Acoustic sensing system for downhole seismic applications utilizing an array of fiber optic sensors

ABSTRACT

A system for sensing subterranean acoustic waves emitted from an acoustic source includes a plurality of laser sources, a plurality of subterranean optical sensors, at least one optical detector, and electronics. The laser sources each emit light at a different frequency. The subterranean optical sensors receive the light and alter the light in response to the acoustic waves. The optical detector receives the altered light and outputs an electrical signal. The electronics receives the electrical signal and converts it into seismic data format. Preferably, the light emitted from the optical sources is modulated at a plurality of modulation frequencies. The electronics can be used to demodulate the signal. The electronics may demodulate the electrical signal by mixing the signal with periodic waveforms having frequencies corresponding to the modulation frequencies and twice the modulation frequencies. The modulation frequencies are selected such that at least one of the second harmonic frequencies associated with the modulation frequencies is interleaved in a non-interfering manner within the corresponding set of first harmonic frequencies. Preferably, the modulation frequencies are selected such that at least one of the first harmonic frequencies is interleaved in a non-interfering manner within the corresponding set of modulation frequencies. The hydrophone for sensing the acoustic signals is able to operate at pressures of at least 5,000 psi and temperatures of at least 130 degrees Celsius. The hydrophone may be housed in a cable having a diameter of less, than about 1.5 inches. The hydrophone&#39;s sensor preferably includes a reference mandrel, two sensing mandrels, and a telemetry can, all of which are aligned in a coaxial, end-to-end configuration to reduce the profile of the hydrophone. Flexible interlinks having grooves therein for receiving optical fiber join the mandrels together. The reference mandrel and sensing mandrels advantageously have hemispherically-shaped endcaps, permitting them to operate at high pressure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to acoustic sensing systems, andmore specifically relates to a system for sensing acoustic wavescomprising an acoustic sensor array.

2. Description of the Related Art

Typically, to obtain oil, a well or hole is dug by drilling and removingearth from the ground to form a shaft known as a “borehole,” whichextends to the bottom of the well. Generally, a large metal pipe orcasing will be inserted into the borehole. Smaller pipes, known asproduction tubes, are inserted into the casing. These production tubesallow access to the bottom of the well. For example, oil may be drawnfrom the well through the production tubing.

Ultimately, the well will appear to go dry. Despite the apparent lack ofoil within the well, vast supplies of oil are often trapped in pocketsin the earth nearby the well. These pockets, however, are generallyinaccessible to the drilled well. To locate such pockets, known in theart as “in-place” reserves, geologists conduct surveys of swaths ofearth surrounding the wells. Geologists employ techniques likecross-well tomography in which acoustic waves are transmitted through avolume of earth to characterize properties, such as density, in thatvolume. Knowledge of the density of the earth helps determine thepresence or absence of oil in the region of the earth beingcharacterized.

To survey the transmission characteristics of a region of the earth, anacoustic wave source can be used to generate acoustic waves, i.e.,sound, while an array of acoustic sensors detects these acoustic waves.Generally, each of the sensors in the array will be situated at adifferent location. The acoustic waves emitted from the acoustic sourceare thus sampled at a plurality of points which typically make up aline. By changing the location of the acoustic source, the location ofthe sensor array, or both, the transmission characteristics of a volumeof earth may be measured. In this manner, a three-dimensional map of thedensity throughout a region of earth can be produced.

Although some prior art techniques rely on acoustic sources and/orsensor arrays situated on the surface of the earth, placing the acousticsources and sensor arrays deep within the earth is more effective forsurveying lower regions of the earth. To conduct measurements deepwithin the earth, a probe can be lowered into the well.

However, the frailty of conventional prior art sensors prevents priorart sensor arrays from being employed deep within a well. Conventionalsensor arrays employ piezoelectric transducers (or piezos) to convertvibrations originating from the acoustic waves into electronic signals.Since a piezoelectric transducer outputs only a small signal, anelectronic preamplifier must be mounted near the piezo to prevent noisefrom overwhelming the small transducer signal. Electronics, however, areincompatible with the harsh environmental conditions, such as hightemperature and pressure, that prevail deep within the earth. Evenpreamplifiers designed to survive high temperature have a short lifetimeand may last, for example, only for one hour under harsh conditions.Thus, the requirement for an electronic preamplifier preventspiezoelectric transducers from being employed deep within a well.

Fiber optic sensors, on the other hand, are electrically passivedevices. That is, they do not require electrical components or externalelectrical connections. Thus they are less susceptible to the harshnessassociated with high temperature, high pressure environments.Furthermore, fiber optic sensors avoid the environmental problemsassociated with electrical components, e.g., the electromagneticinterference that arises when electrical components are placed in thepresence of transmission lines. For these reasons, fiber optic sensorsare sometimes used in hydrophones operating under harsh environmentalconditions.

Fiber optic hydrophones can generally be classified into two categories.Hydrophones of the air backed mandrel design have a hollow, sealedcavity that deforms in response to acoustic pressure, so that strain istransferred to the fiber wrapped around the mandrel. Other, lesssensitive, fiber optic hydrophone designs record the effects of pressuredirectly on the fiber itself, e.g., the fiber may be wrapped around asolid body. Fiber optic hydrophones with high sensitivity (i.e., airbacked mandrel hydrophones) are generally limited to operating pressuresof less than about 5000 pounds per square inch (psi) and temperatures ofless than about 120° C. Outside this range, the materials used in themandrels of air backed mandrel hydrophones deform excessively. Forexample, polycarbonate plastic deforms at these temperatures, whereasmetals such as aluminum buckle inelastically when subjected to highpressures. On the other hand, fiber optic hydrophones utilizing solidbodies or fiber for acoustic transduction typically have much lowersensitivities.

In addition to operating limitations on pressure and temperature,current fiber optic hydrophones are generally bulky, and may have largecross sections that do not lend themselves to use in applications wherecompactness is essential, e.g., in commercial petrochemical wells andboreholes. Thus, there is a need for a fiber optic hydrophone having arelatively small cross section and the ability to withstand highpressures and temperatures.

In addition to restrictions on the placement of the prior art acousticarrays, limitations exist on the number of sensors that may be employedin prior art acoustic arrays. With a larger number of sensors moreinformation must be processed. Limitations on the amount of informationthat can be processed within a reasonable amount of time restrict thenumber of sensors that can be used. Higher resolution maps, however, canbe achieved with a larger number of sensors.

Thus, a need exists for a system for sensing acoustic waves that isrugged enough to operate in the harsh downhole environment andaccommodates a large number of sensors.

Systems accommodating a large number of sensors may benefit from the useof multiplexing, in which multiple signals are communicated within asingle line. One common approach, known as frequency divisionmultiplexing (FDM), operates by modulating a carrier wave at a number ofdifferent frequencies equal to the number of signals that are to bemultiplexed. When FDM is applied to a system using interferometricsensors, the multiplexed signal includes signal components not just atthe modulation frequencies, but at all harmonic frequencies of themodulation frequencies as well. For such a system, the multiplexedsignal may be demultiplexed through detection of the signal componentsat the modulation and first harmonic frequencies, provided thesecomponents do not overlap (in frequency) one another or any componentsat the higher harmonics. Such overlap may be prevented by selectingmodulation frequencies that are sufficiently large and separated thatthe lowest second order harmonic component exceeds the highest firstharmonic component. This leads to large bands of unused frequencybetween DC and the highest frequency signal component detected. However,to keep the signal processing electronics simple it is preferable tokeep the maximum frequency detected as low as possible. Thus, a needexists for a method of selecting a set of FDM modulation frequencieshaving as low a maximum frequency as possible while maintainingfundamental and first harmonic signal components that are not overlappedby other signal components.

SUMMARY OF THE INVENTION

One embodiment of the present invention comprises an electronicinstrument for processing a plurality of optical signals produced by aplurality of optical sensors that sense subterranean acoustic waves. Theelectronics comprise a plurality of optical detectors that convert theoptical signals into electrical signals. An interface outputs a signalderived from the electrical signal in seismic data format.

Another electronic instrument for processing a plurality of modulatedoptical signals produced by a plurality of optical sensors that sensesubterranean acoustic waves is also disclosed. The electronics comprisea plurality of optical detectors that convert the optical signals intomodulated electrical signals. The electronics further comprises at leastone mixer for mixing at least one of the modulated electrical signalswith a carrier. A demodulated signal is thereby generated. An interfaceoutputs a signal derived from the demodulated signal in seismic dataformat.

In another preferred embodiment, an electronic instrument for processinga plurality of modulated optical signals produced by a plurality ofoptical sensors that sense subterranean acoustic waves comprises aplurality of optical detectors. The optical detectors convert theoptical signals into modulated electrical signals. The electronicinstrument further includes a plurality of channels, each of whichcomprises one mixer for mixing one of the modulated electrical signalswith a carrier. A first demodulated signal is thereby generated. Aninterface outputs a signal derived from the demodulated signal inseismic data format.

Another embodiment also directed to an electronic instrument forprocessing a plurality of optical signals produced by a plurality ofoptical sensors that sense subterranean acoustic waves comprises meansfor converting the optical signals into electrical signals. Theelectronic instrument further comprises means for outputting a signalderived from the electrical signal in seismic data format.

A method for processing a plurality of modulated optical signalsproduced by a plurality of optical sensors that sense subterraneanacoustic waves is also disclosed. The method includes the step ofconverting the optical signals into modulated electrical signals. Atleast one of the modulated electrical signals is mixed with a carrierthereby generating a demodulated signal. A signal is derived from thedemodulated signal and is outputted in seismic data format.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in detail below in connectionwith the attached drawings, in which:

FIG. 1 illustrates a side elevational view of a downhole acousticsensing system that is the preferred embodiment of the presentinvention;

FIG. 2 illustrates a perspective view of a cable comprising a downleadcable and a sensor array cable;

FIG. 3A illustrates a schematic view of the first embodiment of theacoustic sensing system of the present invention comprising six lasersources, sixteen optical detectors, and 96 acoustic sensors, wherein thesensors are contained within a single acoustic sensor array;

FIG. 3B illustrates a schematic view of an embodiment of the acousticsensing system of the present invention comprising six laser sources, 32optical detectors, and 192 acoustic sensors, wherein the sensors arecontained within two separate acoustic sensor arrays;

FIG. 4, comprising FIGS. 4A-4H, illustrates a schematic view of oneimplementation of the distribution and return of the optical signal inthe first embodiment. This implementation accommodates a 6×16 opticalsensors array having sixteen sensor groups, wherein each sensor grouphas a dedicated return fiber line;

FIG. 5 illustrates a schematic view of one preferred embodiment of theacoustic sensor, a fiber sensor that is a Mach-Zehnder interferometer;

FIG. 6 illustrates a block diagram of the detector/electronics assemblyand laser drawer in the first embodiment of the acoustic sensing systemhaving 96 sensors in the 6×16 sensor array of FIG. 4;

FIG. 7 illustrates a flow chart of the interaction of the acousticsource and the acoustic sensing system;

FIG. 8 illustrates a flow chart of the operation of the acoustic sensingsystem, namely, the process by which acoustic waves are sensed and datais output in conventional industry standard seismic format; and

FIG. 9, comprising FIGS. 9A-9B, illustrates a schematic view of thedetector/electronics assembly and laser drawer in the second embodimentof the acoustic sensing system having 192 sensors in a 2×(6×16) sensorarray.

FIG. 10, comprising FIGS. 10A and 10B, illustrates frequency componentsfor multiplexed signals in which the modulation frequencies have beenselected so as to keep the fundamental, first harmonic, and secondharmonic sets from overlapping. FIGS. 10A and 10B illustrate thecomponents for systems with five and six modulation frequencies,respectively.

FIG. 11, comprising FIGS. 11A and 11B, illustrates frequency componentsfor multiplexed signals in accordance with an embodiment of the presentinvention, wherein the modulation frequencies are selected to be equallyspaced, and wherein the first harmonic and second harmonic sets overlapwithout overlapping the component signals within the two sets. FIGS. 11Aand 11B illustrate the components for systems with five and sixmodulation frequencies, respectively.

FIG. 12 illustrates frequency components for a multiplexed signalresulting from five light sources in accordance with an embodiment ofthe present invention, wherein the modulation frequencies are evenlyspaced beginning at 6 Δf, except for skipping a modulation frequency at9 Δf.

FIG. 13 illustrates frequency components for a multiplexed signalresulting from six light sources in accordance with an embodiment of thepresent invention, wherein the modulation frequencies are evenly spacedbeginning at 7 Δf except for skipping a modulation frequency at 12 Δf.

FIG. 14, comprising FIGS. 14A and 14B, illustrates frequency componentsfor a multiplexed signal resulting from six light sources in accordancewith an embodiment of the present invention, wherein the modulationfrequency components are selected at Δf multiples of 5⅔, 7, 8, 9, 10,and 12½. For clarity, FIG. 14A isolates the fundamental frequencycomponents.

FIG. 15, comprising FIGS. 15A and 15B, illustrates frequency componentsfor a multiplexed signal resulting from six light sources in accordancewith an embodiment of the present invention, wherein the modulationfrequency components are selected at Δf multiples of 3, 4, 5, 7, 11, and13. For clarity, FIG. 15A isolates the fundamental frequency components.

FIG. 16 illustrates a cutaway view of a hydrophone embodiment thatresides within a cable.

FIG. 17 illustrates a cross sectional view of the cable of FIG. 16 at alocation away from the hydrophone.

FIG. 18 illustrates mechanical support features used around thehydrophone's sensor to protect it from breakage that might otherwiseoccur during bending of the cable.

FIG. 19 illustrates an expanded view of the sensor showing a telemetrycan, a reference mandrel, and two sensing mandrels, as well as theoptical fibers that link them.

FIG. 20, comprising FIGS. 20A, 20B, and 20C, illustrates schematicdiagrams of the optical fiber routing within the sensor. In FIGS. 20A,20B, and 20C, the sensor functions as a Mach-Zehnder interferometer, aMichelson interferometer, and a Fabry-Perot interferometer,respectively.

FIG. 21 illustrates a perspective view of the reference mandrelincluding its hemispherical endcaps.

FIG. 22 illustrates a cross sectional view of a hemispherical endcap.

FIG. 23 illustrates a flexible interlink used to join two hemisphericalendcaps.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A system 100 for sensing acoustic waves 102 in accordance with apreferred embodiment of the present invention is shown in FIG. 1. Thesystem 100 comprises an acoustic array cable 104 attached to a downleadcable 106 which is held on a first spool 108 on a first truck 110. Thedownlead cable 106 passes from the first spool 108 to a reel 112, alsomounted on the first truck 110, and to a sheave 114 situated on asurface 116 adjacent to a well 118. From the sheave 114, the downleadcable 106 runs up to a pulley 120 fixed to a crane 122. The downleadcable 106 and the acoustic array cable 104 extend from this pulley 120into the well 118. The well 118 comprises a first borehole 124 formed ina layer of earth 126. A large metal pipe known as a casing (not shown)is inserted into the borehole 124. The downlead cable 106 on the spool108 is connected to a receiver processing electronics 128 housed in thefirst truck 110.

An acoustic source 130 is situated in a second borehole 132. Thisacoustic source 130 is attached to an acoustic source cable 134, whichis held on a second spool 136 on a second truck 138. The acoustic sourcecable 134 passes from the second spool 136 to a second reel 140, alsomounted on the second truck 138, and to a second sheave 142 situated onthe surface 116 adjacent to the second borehole 132. From the secondsheave 142, the acoustic source cable 134 runs up to a second pulley 144fixed to a second crane 146. The acoustic source cable 134 extends fromthis pulley 144 into the second borehole 132. Also housed in the secondtruck 138 is source electronics 148 associated with the acoustic source130. The acoustic waves 102 emanate from the acoustic source 130 in thesecond borehole 132 and arrive at the acoustic array cable 104 in thefirst borehole 124.

A perspective view of a cable 202 comprising the downlead cable 106 andthe acoustic array cable 104 is shown in FIG. 2. An interface 204connects the downlead cable 106 to the acoustic array cable 104. Theacoustic array cable 104 is terminated by a gamma detector 206, whichoperates in a conventional manner to produce an electrical signalresponsive to the passage of the gamma detector 206 through each sectionof pipe forming the casing within the borehole 124. The gamma detector206 provides a signal that is a processed to determine the depth to thetermination of the acoustic array cable 104.

As shown in FIG. 3A, a plurality of laser sources LS1, LS2, LS3, LS4,LS5, LS6 are positioned to supply optical feed lines F1-F6, which arejoined at an optical terminator 302. The optical terminator 302 connectsto the downlead cable 106, which is connected to the acoustic arraycable 104. The acoustic array cable 104 houses a plurality of sensors,which in this exemplary embodiment total 96 and are designated S1-S96.The optical terminator 302 also provides a link between the downleadcable 106 and a plurality (e.g., 16) of return fibers R1-R16, which arecoupled to optical detectors D1-D1 6. The outputs of the opticaldetectors D1-D16 are electrically connected to processing electronics304.

Each laser source LS1, LS2, LS3, LS4, LS5, LS6 comprises a respectivelaser L1 L2, L3, L4, L5, L6 and a modulator M1, M2, M3, M4, M5, M6. Eachof the lasers L1-L6 generates an optical beam having a different opticalwavelength. The six optical beams produced by these lasers L1-L6 aredirected to respective modulators M1-M6. Preferably, these modulatorsM1-M6 comprise phase modulators, each characterized by a differentmodulation frequency. Accordingly, the laser sources LS1, LS2, LS3, LS4,LS5, LS6 output six optical signals each having different opticalwavelengths and each modulated at a separate modulation frequency.

FIG. 3B shows an embodiment comprising 192 sensors S1-S192 containedwithin two separate acoustic array cables 104 a, 104 b appended to twoseparate downlead cables 106 a, 106 b. The two separate acoustic arraycables 104 a, 104 b and downlead cables 106 a, 106 b could be insertedin two separate boreholes 124. This embodiment having 192 sensors willbe discussed more fully below.

The plurality of feed lines F1-F6 are connected to a plurality ofdistribution fiber lines DF1-DF6 (shown in FIGS. 4A-4H) at the opticalterminator 302 to transfer the optical signals outputted by the lasersources LS1-LS6 to the distribution fiber lines. These distribution feedlines DF1-DF6 run through the downlead cable 106 and into the acousticarray cable 104 as well.

FIG. 4, which comprises FIGS. 4A-4H, shows the 96 sensors S1-S96 in asingle acoustic array cable 104 similar to that shown in FIG. 3A. These96 sensors S1-S96 are divided into eight sensor groups of twelve sensorseach. A first sensor group, group 401, is shown in FIG. 4A. The opticalpath from the first sensor group 401 to the laser sources LS1, LS2, LS3,LS4, LS5, LS6 and to the processing electronics 304 is shorter than forany of the other sensor groups 402-408. Seven additional sensor groups402-408 are shown in FIGS. 4A-4H. Each sensor group 401-408 has at leastone sensor coupled to each of the six distribution fiber lines DF1-DF6.For example, in the first sensor group 401, the distribution fiber linesDF1-DF6 are connected to respective standard 1×2 input couplers 420,which are in turn connected to respective sensors S1-S12. Similarly, inthe second sensor group 402, the distribution fiber lines DF1-DF6 areconnected to respective sensors S13-S24 via additional standard 1×2input couplers 420.

All the sensors S1-S12 in the group 401 are coupled to two return fiberlines RF1, RF2. Similarly, each of the sensor groups 402-408 has two ofthe return fiber lines RF2-RF16 dedicated solely to its use. Forexample, sensors S7-S24 are all coupled to two of the return fiber linesRF1-RF16, namely, the third and fourth fiber lines RF3, RF4. As afurther example, the sensors S85-S96 are coupled to the last two fiberlines RF15, RF16. In this embodiment, no adjacent sensors S1-S96 share acommon return fiber line RF1-RF16.

The return fiber lines RF1-RF16 are connected to return fibers R1-R16.The return fiber lines RF1-RF16 and the return fibers R1-R16 direct theoptical outputs of the acoustic sensors S1-S96 to the optical detectorsD1-D16.

In FIG. 5, the acoustic sensors S1-S96 comprise an interferometer 502that is sensitive to acoustic pressure, pressure changes, or pressurewaves. The interferometer 502 depicted in FIG. 5 is a Mach-Zehnderinterferometer. This interferometer 502 includes a sensor input line504, which is connected to a first coupler 506. A reference arm 508 anda test or sensing arm 510 are attached to this first coupler 506. Thereference arm 508 and the test arm 510, are optical fibers. The opticalfibers 508, 510 are connected to a second coupler 512 that is connectedto a sensor output line 514. The input coupler 420 and output coupler430 are connected to the sensor input line 504 and sensor output line514, respectively.

The optical signal that emanates from the laser sources LS1-LS6 iscoupled into the sensor input line 504 of the interferometer 502 via theinput coupler 420. This signal is split by the first coupler 506 intotwo beams. A reference beam travels through the reference arm 508, and atest beam travels through the test arm 510. The two beams are coupledinto a single fiber 514, the sensor output line, at the second coupler512 of the interferometer 504. The reference beam and the test beaminterfere in the second coupler 512 to produce an output signal that isdetected at one of the optical detectors D1-D16.

Acoustic vibrations that impinge on one of the acoustic sensors S1-S96cause the optical fiber comprising the respective test arm 510 to bedeformed, e.g., to be stretched or contracted, which in turn changes theoptical path length of the test arm 510. In contrast, the reference arm508 is shielded from the acoustic vibration. Thus, the optical pathlength of the reference arm does not change. Since the optical pathlength of the test arm 510 changes while the optical path length of thereference arm 508 does not change, the phase difference between thebeams traveling in the test and reference arms changes in response tothe acoustic vibrations. The changes in relative phase between the testand reference arms 510, 508 result in time-varying interference at thesecond coupler 512. The time-varying interference results in a timevarying light intensity of the signal output from the second coupler512. The time-varying light intensity is detected by one of thedetectors (e.g., the first detector D1).

FIG. 6 depicts a detector/electronics assembly 601 for the firstembodiment of the acoustic sensing system 100, which has sixteen returnfibers R1-R16 that are coupled to the sixteen optical detectors D1-D16.The detector/electronics assembly 601 includes the optical detectorsD1-D16 and the processing electronics 304.

FIG. 6 also schematically shows an optic sensor array 602 andillustrates how the detector/electronics assembly 601 is connected tothe optical sensor array and to the laser sources LS1-LS6. As definedherein, the optical sensor array 602 comprises a plurality of opticalsensors coupled together using optical fibers. The optical sensor array602 shown in FIG. 6 includes the designation 6×16 corresponding to thesix distribution fiber lines DF1-DF6 and 16 return fiber lines RF1-RF6shown in FIGS. 4A-4H.

Each of the optical detectors D1-D16 is included as part of the four24-channel digital receivers/demodulators 604. The optical detectors areseparated into four groups, D1-D4, D5-D8, D9-D12, and D13-D16, whereineach group is situated in one of the four 24-channel digitalreceiver/demodulators 604.

As shown in FIG. 6, the four 24-channel digital receiver/demodulators604 are electrically connected to four 24-channel digital signalprocessors (DSPs) 606. Each of the 24-channel DSPs 606 comprises twelvedigital signal processing chips. Accordingly, the term “12-DSPprocessing element” 606 may be used interchangeably with 24-channeldigital signal processors.

Each of the 24-channel digital receiver/demodulators 604 is paired withone of the 12-DSP processing elements 606. The four 12-DSP processingelements are coupled to a PCI bus 608 (or other suitable bus), which iscoupled to a central processing unit (CPU) 610, such as, for example, anIntel Pentium II or Pentium III processor.

The CPU 610 is coupled to a hard drive 612 via a SCSI bus 614. Thecentral processing unit 610 is also connected to an operator console 616and a recording and processing system 618 via two Ethernet lines 620,622.

Each of the 24-channel digital receiver/demodulators 604 accommodates 24signals because each of the four detectors D1-D16 within one of thedigital receiver/demodulators receives six signals from a group of sixsensors. The six signals that arrive at each of the optical detectorsD1-D16 originate from the six laser sources LS1-LS6 and have a differentoptical wavelength and have different modulation frequency. Upon beingirradiated by the six signals, each of the optical detectors D1-D16outputs an electrical signal having components proportional to theintensity of the optical light incident thereon at each of themodulation frequencies and at harmonics of the modulation frequencies.The electrical signal from one of the optical detectors, e.g., the firstdetector D1, is separated into the six signals produced by the sixacoustic sensors, e.g., the first six odd sensors S1, S3, S5, S7, S9,S11, whose outputs are channeled to the optical detector. The sixsignals are distinguished by separating the components according to themodulation frequencies. Although the light incident on the detector D1comprises six different optical wavelengths, it is not necessary toseparate the signals optically. The difference in optical wavelengths isused to keep the six signals from optically interfering with each other.

The total number of acoustic sensor signals processed by thedetector/electronics assembly 601 employed in the embodiment depicted inFIG. 6 is 96. Each of the 24-channel digital receiver/demodulators 604receives four optical signals from four of the return fibers R1-R16. The24-channel digital receiver/demodulator 604 converts each of the fouroptical beams into six separate electrical channels, resulting in 24electrical channels. Since the detector/electronics assembly 601 for theembodiment shown in FIG. 6 has four 24-channel digitalreceiver/demodulators 604, a total of 96 (4×24) electrical channels areutilized. Each of the 96 electrical channels contain informationrelating to the acoustic vibrations at a respective one of the 96acoustic sensors S1-S96.

As noted above, each of the acoustic sensors S1-S96 comprises aninterferometer 502 that splits the coherent light source into two wavesfollowing separate paths that eventually converge. Upon convergence, thetwo waves interfere with each other such that the intensity I of thecombination is given by I=A+B cos θ, where A and B are constants and θis the phase difference between the two waves upon convergence.

In order to multiplex the six sensor signals associated with the sixlasers L1-L6 that are transmitted via each return fiber (e.g., RF1), theinterferometer phase angle of each of the six sensors is modulated at adifferent frequency, ω_(n). The interferometer phase angle modulationmay be represented as θ(t)=C_(n) cos ω_(n)t, where n=1, . . . , 6, andC_(n) is the amplitude of the phase modulation in radians. The phaseangle in the interferometer is modulated by sinusoidally varying thephase of each laser L1-L6. This is accomplished by the modulator M1-M6by sinusoidally varying the voltage across a lithium niobate segment(not shown) of the optical path. A laser source phase modulation, Φ=Φ₀cos(ωt), where Φ₀ is the phase amplitude in radians, results in a laserfrequency modulation f=f_(c)+Δf sin(ωt), where f_(c) is the opticalcarrier frequency and Δf=Φ₀ω/2π. This frequency modulation, in turn,results in a modulation of the interferometer phase angle, φ=2πΔLΔf/csin(ωt), where ΔL is the path length offset between the twointerferometer paths and c is the speed of light in the fiber.

This modulation results in a time varying intensity for the output ofthe nth interferometer given by: I_(n)(t)=A_(n)+B_(n) cos[C_(n)cos(ω_(n)t)+φ_(n)(t)], where φ_(n)(t) is the time varying phase createdby the acoustical signal in the nth optical sensor (and signal noise).This equation may be expanded in terms of Bessel functions to give:${I_{n}(t)} = {A + {B_{n}\left\{ {{{〚{{J_{o}\left( C_{n} \right)} + {2{\sum\limits_{{k = 1},\infty}^{\quad}\quad {\left( {- 1} \right)^{k}\quad {J_{2k}\left( \quad C_{n} \right)}\quad \cos \left( \quad {2\quad k\quad \omega_{n}t} \right)}}}}〛}{\cos\left( \quad {\phi_{n}(\quad t)} \right)}} - \quad \quad {\left. 〚{{{2\quad {\sum\limits_{{k = 0},\infty}^{\quad}\quad {\left( {- 1} \right)^{k}{J_{{2k} + 1}\left( C_{n} \right)}\quad {\cos \left( {\left( {{2k} + 1} \right)\quad \omega_{n}t} \right)}}}}〛}{\sin \left( {\phi_{n}(t)} \right)}} \right\}.}} \right.}}$

As noted earlier, the N lasers L1-L6 are chosen to have sufficientlydifferent optical carrier frequencies to avoid optical interference.Thus, the total intensity on the detector, I_(tot), connected to thisparticular return fiber (e.g., RF1) is then given by${I_{tot}(t)} = {\sum\limits_{{n = 1},6}\quad {{I_{n}(t)}.}}$

The light intensities detected by each of the 16 detectors D1-D16 isdescribed by an analogous equation.

The above equations demonstrate that the interferometer intensity outputcontains signal not only at the six modulation frequencies ω_(n), butalso at 2ω_(n), 3ω_(n), etc. The multiplexed intensity signal receivedby a given detector D1-D16 may be fully demultiplexed through detectionof the signal components at ω_(n) and 2ω_(n) using the followingapproach.

The total output signal, I_(tot), may be mixed with a signal at ω_(n)and a signal at 2ω_(n), and the results of the mixing may be low passfiltered to remove the signal at all harmonics above the first harmonic.This results in “direct” (I) and “quadrature” (Q) components, such that:I_(n)=B_(n)GJ₁(C_(n)) sin φ_(n)(t) and Q_(n)=B_(n)HJ₂(C_(n)) cosφ_(n)(t), where G and H are the amplitudes of the mixing signalscorresponding to the ω_(n) and 2ω_(n) components of the signal,respectively. The properties of Bessel functions are such that J₁(x) andJ₂(x) are equal when the parameter x≈2.6. See, e.g., Handbook ofMathematical Functions, 1974, edited by M. Abramowitz and I. Stegun.Then, by choosing G=H and C_(n)=2.6 radians, the phase angle is givenby: φ_(n)(t)=arctan(I_(n)/Q_(n)).

Thus, to demodulate, the 24-channel digital receiver/demodulators 604mix the electrical signals output by the optical detectors D1-D16 withsinusoidal waveforms at the six frequencies at which the output of thesix lasers L1-L6 are modulated. The 24-channel digitalreceiver/demodulators 604 also mix the electrical signals output by theoptical detectors D1-D16 with sinusoidal waveforms having twice thesesix frequencies. Accordingly, the 24-channel digitalreceiver/demodulators 604 will mix the electrical signals output by theoptical detectors D1-D16 with sinusoidal carriers at frequencies of ω₁,ω₂, ω₃, ω₄, ω₅, and ω₆, and 2ω₁, 2ω₂, 2ω₃, 2ω₄, 2ω₅, and 2ω₆.

As noted above, the demodulated signals produced as a result of thismixing result in direct (I) and quadrature (Q) components. Thesecomponents are provided for each channel as inputs to a circuit (notshown) that outputs the arctangent of the two components. In thismanner, polar phase is obtained from the demodulated signals. This polarphase corresponds to the phase difference between the optical beams inthe test and reference arms 510, 508. The time derivative of the polarphase is generated from digital circuitry (not shown) that is designedto implement differentiation. The derivative of the phase isproportional to the magnitude of the acoustic vibrations sensed at thesensors S1-S96.

The derivative of the phase produced by two channels of each 24-channeldigital receiver/demodulator 604 is sent to one element of thecorresponding 12-DSP elements 606. The 12-DSP elements 606 filter anddecimate the demodulated signals down to standard sample rates requiredby conventional seismic data recorders. These 12-DSP elements 606 arecoupled to the PCI bus 608 and use the PCI bus to communicate with theCPU 610. Accordingly, the filtered and decimated derivative of the phaseare fed into the CPU 610. Note that each of the 12-DSP elements 606processes the phase information from two acoustic channels, each ofwhich is performed separately.

The CPU 610 formats the data corresponding to the acoustic vibrationssuch that it is compatible with industry standards (e.g., the SEG-Dformat). For example, the CPU 610 stamps the acoustic data output withthe time of system events such as the start of sensing. The CPU alsoadds any necessary information to identify the data in accordance withthe industry standard format.

The CPU also handles interfaces with conventional seismic data recordingequipment. The CPU 610 sends the reformatted acoustic data to seismicdata recording equipment at industry standard data rates. Morespecifically, the processed and formatted signals generated from theacoustic sensors S1-S96 and optical detectors D1-D16 are transmittedover the PCI bus 608 to the CPU 610 and are outputted to customersupplied seismic processing equipment via the Ethernet line 622.

The host CPU 610 additionally provides system control and sequencing forthe operation of the individual components in the acoustic sensingsystem 100.

The CPU also handles interfaces with an operator console 616. Theoperator console 616 allows manual system intervention and is also usedto display system status.

The detector/electronics assembly 601 additionally includes an auxiliaryinput/output subsystem 624 that interfaces with the central processingunit 610 via the PCI bus 608. This auxiliary input/output subsystem 624interface with customer supplied equipment (CSE) 626 to provide up tosixteen acoustic or non-acoustic sensor inputs for time marking or eventtriggering.

The detector/electronics assembly 601 additionally includes a globalposition sensing (GPS) electronics card 628 that is electronicallyconnected to an antenna 630. The GPS electronics card 628 interfaceswith the CPU 610 via the PCI bus 608. The GPS electronics card 628provides accurate time for the host CPU 610 to facilitate time stampingof system events.

In the embodiment shown in FIG. 6, a frequency synthesizer card 632 isincluded with the detector/electronics assembly 601. The frequencysynthesizer card 632 accepts a sync pulse from additional customersupplied equipment (CSE) 634. Preferably, the frequency synthesizer card632 accepts a sync pulse from the source electronics 148 associated withthe acoustic source 130 in FIG. 1. As shown in FIG. 1, the electronics148 associated with the acoustic source 130 is located in the secondtruck 138 adjacent the second borehole 132.

The frequency synthesizer card 632 is electrically connected to a lasermodule controller/driver card 636, which is connected to the lasersources LS1-LS6, both of which are preferably located in a laser drawer638. Additionally, the frequency synthesizer card 630 is electricallyconnected to an ISA bus 640 that is also coupled to the centralprocessing unit 610.

As described above, the laser sources LS1-LS6 include lasers L1-L6 andmodulators M1-M6, which provide signals to the optical feed lines F1-F6that are coupled to the acoustic sensors S1-S96. The frequencysynthesizer card 632 provides the modulators M1-M6 with periodicwaveforms having the six modulation frequencies to modulate the outputsof the six lasers L1-L6. The frequency synthesizer card 632 alsoprovides the 24-channel digital receiver/demodulators 604 with globalsynchronization and timing signals to insure that the modulators M1-M6and demodulator are phase locked. In particular, the frequencysynthesizer card 632 provides a sync signal and a high speed clocksignal to the 24-channel digital receiver/demodulators 604. Using thissync signal and this clock signal, the 24-channel digitalreceiver/demodulators 604 generate digital representations of sinusoidalcarriers at the six modulation frequencies ω₁, ω₂, ω₃, ω₄, ω₅, ω₆ and attwice the modulation frequencies 2ω₁, 2ω₂, 2ω₃, 2ω₄, 2ω₅, and 2ω₆. Thesedigital carriers are employed by 24-channel digitalreceiver/demodulators 604 for mixing and demodulation as describedabove.

The operation of the above-described acoustic sensing system 100 aspresented in FIGS. 1-6 is illustrated in FIG. 7 in flowchart form. Afirst block 702 in a source flow diagram represents the triggering eventfor the operation of the acoustic sensing system 100, wherein theacoustic source 130 transmits a sync pulse to the acoustic sensingsystem. (See FIG. 1.) In an alternative preferred embodiment, theacoustic sensing system 100 can send a sync pulse to the acoustic source130 to trigger the source. This acoustic source 130 may comprise, e.g.,a surface acoustic source or an underground acoustic source.

The acoustic sensing system 100 receives the sync pulse as indicated bya first block 704 in a series of blocks corresponding to the stepsperformed by the acoustic sensing system 100. In response to receivingthe sync pulse, the acoustic sensing system 100 begins sensing. That is,the acoustic sensing system 100 begins measuring the level of acousticvibration at the sensors S1-S96. The start of the sensing is representedby block 706 in FIG. 7.

As shown in the source flow diagram, after a predetermined delay (block708), the acoustic source 130 starts producing acoustic waves 102 asindicated in a block 710. As represented by a block 712, the acousticsensing system 100 continues monitoring the level of acoustic vibrationat the sensors S1-S96 and begins to sense the acoustic waves 102 emittedby the acoustic source 130 that reach the acoustic sensors. A moredetailed discussion of the steps involved in sensing acoustic vibrationare presented in FIG. 8 in flow chart form, as discussed more fullybelow.

A block 714 represents the sensing system 100 sending the results ofmeasurements of the level of vibration at the acoustic sensors S1-S96 toseismic processing system as seismic data. At a block 716, the system100 stops sensing the acoustic data. A determination as to when to stopsensing data is advantageously based upon the expiration of apredetermined time internal from the sync pulse.

The process for sensing acoustic data in the block 706 and the block 712in FIG. 7 is depicted in more detail in FIG. 8. As discussed above, thesensing for acoustic vibration at the acoustic sensors S1-S96 startsimmediately after receiving the sync pulse, although a delay existsbetween the time the sync pulse is received and the acoustic source 130begins producing acoustic waves 102. This permits the seismic processingsystem to receive data indicative of the acoustic background noise priorthe receipt of acoustic waves from the acoustic source.

In FIG. 8, a first block 802 indicates that continuous wave light isemitted from each of the laser sources LS1-LS6. The light from eachsource is modulated, as discussed above. In particular, the light fromeach of the laser sources LS1-LS6 is modulated at a different modulationfrequency.

A block 804 represents the next step wherein the distribution fiberlines DF1-DF6 propagate the light from the laser sources LS1-LS6 to theoptical sensors S1-S96. As discussed above, the light in the respectivetest arms 508 of the optical sensors S1-S96 is variably delayed whenacoustic waves 102 strike the sensors. (See block 806). The light in thereference arm 510 of each sensor S1-S96 is not variably delayed. Each ofacoustic sensors S1-S96 combines the light from the two arms 508, 510 inthe output coupler 512.

A block 808 represents the return fiber lines RF1-RF16 carrying thelight outputted by the optical sensors S1-S96 to the fiber receivers604, i.e., the 24-channel digital receivers/demodulators 604. The fiberreceivers, which include the optical detectors D1-D16, convert theoptical signals incident on the optical detectors into electricalsignals as indicated in a block 810. As depicted by a block 812, theprocessing electronics 304 convert the electrical signal outputted bythe optical detectors D1-D16 into SEG-D format, a standard formatestablished by the Society of Exploration Geophysicists. The SEG-Dformat is conventional and is well known in the art.

The embodiment described above is particularly well suited forsubterranean geophysical surveys such as are employed in determining thepresence of “in-place” oil reserves. The acoustic sensors S1-S96contained within the acoustic array cable 104 are capable of beinglowered into the borehole of an oil well. The acoustic sensors S1-S96may also be employed for land seismic applications and in ocean bottomcables.

As used herein, the term borehole is defined as a shaft that extends tothe bottom of a well 118 and a “well” is simply a hole dug by drillingand removing earth from the ground, often for the purpose of accessingoil or water.

Cable

The cable 202 shown in FIG. 2 is designed to fit into a well 118 such asan oil well. If the cable 202 is small enough, the cable can be insertedinto the production tubing or in the gaps between the production tubingin the casing. However, the cable needs to be smaller than at least theinner diameter of the production tubing.

As described above, the term “casing” refers to a large metal pipe thatis typically inserted into the borehole. “Production tubes” are smallerpipes inserted in the casing that allow access to the bottom of the well118.

The standard diameter for production tubing is two inches in the UnitedStates and is 1.25 inches in the North Sea. Consequently, to fit in theproduction tubing or in the gaps between the production tubing, thecable 202 needs to have a diameter less than two inches for use in theUnited States and less than 1.25 inches for use in the North Sea.

Conventional electronic acoustic sensor arrays range from 2.5 to 6inches in diameter requiring all the production tubing to be removedfrom the casing in order to insert a probe containing the array downinto the well 118. After the probe is removed, the production tubingmust be reinserted into the casing. The removal and reinsertionprocedure is both costly, time-consuming, and inconvenient.

Accordingly, the cable 202, including the downlead cable 106, theinterface 204, and the acoustic array cable 104 have an outer diameterthat is less than two inches. The diameter of the cable 202 ispreferably than 1.25 inch. More preferably, the diameter of the cable202 is less than 1.1 inches. Also, preferably the diameter of theacoustic array cable 104 does not vary more than ±0.01 inch.

As shown above, the cable 202 includes a downlead cable 106 joined to anacoustic array cable 104. The downlead cable 106 does not contain anysensors S1-S96. Preferably, the downlead cable 106 has a length selectedfrom the range between 1,000 feet and 20,000 feet. In one particularembodiment, the downlead cable 106 is approximately 10,000 feet long.

As described above, the acoustic array cable 104 contains the acousticsensors S1-S96. Preferably, these acoustic sensors S1-S96 are evenlyspaced through the acoustic array cable 104. For example, in oneparticular embodiment each of the acoustic sensors S1-S96 areadvantageously spaced five feet apart within the acoustic array cable104. The spacing, however, may vary ±0.25 inches or by ±0.5% axially.

The spacing in the present invention, however, is not limited tospacings of five feet, rather, the spacing may be larger or smaller thanfive feet. For example, in one application, the acoustic sensors S1-S96may preferably be spaced 5 to 100 feet apart within the acoustic arraycable 104. Closer spacing provides better resolution of the acousticsignals. Greater spacing provides greater coverage of the acousticsignals at the expense of resolution. Although even spacing ispreferable, the spacing need not be the same between each of the sensorsS1-S96. The spacings described above still apply to the case where eachof the sensors S1-S96 are not separated by the same distance.

The length of the active portion of acoustic array cable 104 varies inaccordance with the spacing between the acoustic array sensors S1-S96.The active portion of the array cable 104 is the aperture of the array.Preferably, the acoustic array cable 104 has a length selected from therange between 200 feet and 1000 feet. More preferably, the length of theacoustic array cable 104 is approximately 500 feet. By spacing thesensors farther apart, the aperture can be increased to as much as10,000 feet.

Preferably, the cable 202 is durable enough to protect the distributionfiber lines DF1-DF6, the return fiber lines RF1-RF16, and the acousticsensors S1-S96 against the harsh downhole environment. As used herein,the term “downhole” is defined as down in the borehole. The downholeenvironment includes high temperature and high pressure and may alsoinclude corrosive liquids commonly found in an oil well environment.

In some cases, the cable 202 will be lowered into a pipe such as theproduction tubing or casing in the well where the pressure in a regionof the pipe at the top of the well (i.e., at the surface 116) is higherthan the ambient pressure at the top of the well (i.e., at the surface116 but outside the well). The cable 202 may be lowered through a greaseinjection head capable of maintaining a pressure difference between theambient pressure at the top of the well and the pressure within theregion of the pipe at the top of the well. In the case where the cable202 is lowered through a grease injection head, a cable 202 having auniform diameter is required.

Distribution Fiber Lines

As shown in FIGS. 3 and 4A-4H, the distribution fiber lines DF1-DF6couple the light from the laser sources LS1-LS6 into the optical sensorsS1-S96 via the input couplers 420. In each sensor group 401-408, acertain fraction of the light from the lasers sources LS1-LS6 is coupledto one of the sensors S1-S96 in that group. The amount of light coupledinto each sensor S1-S96 is preferably chosen so as to reduce differencesin the level of optical signal delivered to each sensor, and moreparticularly, to reduce the variations in the power level of the opticalsignals that are delivered to the different optical detectors D1-D16. Adesign for sensor arrays that enables the signal levels of the opticalsignals returned from the sensor groups 401-408 to their associateddetectors D1-D16 to be similar in magnitude is disclosed in the relatedapplication of entitled “Architecture for Large Optical Fiber ArrayUsing Standard 1×2 Couplers”, U.S. patent application Ser. No.09/107,399, filed on Jun. 30, 1998 which is hereby incorporated byreference herein.

Although six distribution fiber lines DF1-DF6 carry light beams emittedby six laser sources L1-L6 as shown in FIGS. 3 and 4A-4H, the number ofdistribution fiber lines that can be used is not restricted to six.Rather, the number of distribution fiber lines DF1-DF6 employed canrange from two to twelve or more. Preferably, however, the number ofdistribution fiber lines DF1-DF6 will correspond with the number oflaser sources LS1-LS6.

Similarly, in the embodiment shown in FIGS. 4A-4H, each of thedistribution fiber lines DF1-DF6 couples light into one of the sensorS1-S96 in each of the sensor groups 401-408. The present invention isnot limited to this arrangement.

Acoustic Sensors

The acoustic sensors S1-S96 that are employed in the embodiment depictedin FIGS. 1-5 are “optical” sensors and more particularly “all-optical”sensors.

As used herein the term “optical” means pertaining to or using light,which corresponds to electromagnetic radiation in the wavelength rangeextending from the vacuum ultraviolet at about 40 nanometers, throughvisible spectrum, to the far infrared at 1 millimeter in wavelength.More particularly, the optical sensors in the present invention operatein the range of visible or infrared wavelengths. Most particularly, theoptical sensors operate in the infrared range at approximately 1319nanometers.

As used herein the term “all-optical” means that the downhole portion ofthe acoustic sensor array does not include any electronics. Inparticular, the acoustic sensors S1-S96 are electrically passivedevices; they require no electrical components or electrical connectionsto the other components. Most notably, the acoustic sensors S1-S96 donot rely on any semiconductor-based electronics, which are highlysensitive to temperature. Semiconductor-based electronics such astransistors are generally not compatible with the high temperatures thatprevail in the downhole environment, e.g., 10,000 feet below the surfaceof the earth. For example, some preamplifiers designed to survive hightemperatures have a short lifetime and may last only for one hour underharsh conditions. In contrast, the embodiment described above requiresno pre-amplifier in the borehole.

Each of the acoustic sensors S1-S96 in the preferred embodimentcomprises a sensor that receives an optical beam as input and thatoutputs an optical signal that contains information corresponding to thelevel of acoustic vibration incident on the sensor. More preferably, thesensors S1-S96 employed in the present invention are fiber-optic sensorswherein a beam of light is inputted into one end of a fiber, the lightbeam is altered in some manner while in the fiber, and this altered beamis outputted at another end of the fiber. As used herein, the termfiber-optic sensor is defined as a sensor for monitoring some physicalproperty that comprises a length of optical fiber having light withinit, wherein the fiber acts as a transducer that modifies some attributeof the light upon exposure to variation in the physical property beingmeasured.

Preferably, the acoustic sensors S1-S96 are optical interferometers.Most preferably the sensors S1-S96 are Mach-Zehnder interferometers.While acoustic sensors S1-S96 as depicted in FIG. 5 compriseMach-Zehnder interferometers, the acoustic sensors of the presentinvention are not so limited but may comprise other interferometers aswell as other types of optical sensors including sensors other thanfiber-optic sensors. Other interferometers may include, for example,Michelson interferometers, Fabry-Perot interferometers, and Sagnacinterferometers.

In accordance with the present invention, the acoustic sensors S1-S96need to be capable of operating in a downhole. In particular, thesensors S1-S96 need to be able to function and output a retrievablesignal at a depth in the range of between 1,000 and 20,000 feet belowthe surface of the earth. More preferably, this depth is approximately10,000 feet.

In particular, the sensors S1-S96 must be capable of functioning withinthe acoustic array cable 104 while the temperature surrounding theacoustic array cable in the range of between 100° C. and 150° C.

Additionally, the sensors S1-S96 must be capable of functioning withinthe acoustic array cable 104 while the pressure on the acoustic arraycable is in the range of 5,500 pounds per square inch (p.s.i.).

The acoustic sensors S1-S96 must be capable of functioning within theacoustic array cable 104 when the acoustic array cable is immersed inwater. Accordingly, the optical sensor S1-S96 may comprise a hydrophone.Alternatively, the optical sensor S1-S96 may comprise a geophone or acombination of a hydrophone and a geophone, e.g., one hydrophone andthree geophones. A geophone is a vector sensor. Consequently thepreferred arrangement is to have three geophones employed together,possibly in combination with a hydrophone.

A hydrophone measures pressure, pressure changes, or both. A hydrophonetypically measures pressure or pressure changes in the audio or seismicrange corresponding to at least 1 Hz to 30 kHz. A geophone measuresmovement, displacement, velocity, and/or acceleration. The geophonetypically measures movement, displacement, velocity, or acceleration inthe audio or seismic range corresponding to at least 0.1 Hz to 10 kHz.One preferred hydrophone design is disclosed below.

Although 96 acoustic sensors S1-S96 are shown in FIGS. 3 and 4A-4H, thenumber of sensors that can be used is not restricted to 96. As describedabove, the number of sensors can be doubled to 192. More generally, thenumber of acoustic sensors S1-S96 can range from two to more than 200.If time division multiplexing is also employed, the number of acousticsensors S1-S96 can be increased 10 to 100 times. Accordingly, the numberof acoustic sensors S1-S96 can range from two to 20,000 or more.Preferably, however, the number of acoustic sensors S1-S96 correspondsto the product of the number of laser sources LS1-LS6 and the number ofoptical detectors D1-D16 which also corresponds to the product of thenumber of distribution fibers lines DF1-DF16 and the number of returnfiber lines RF1-RF16.

Return Fiber Lines

As shown in FIGS. 3 and 4A-4H, the return fiber lines RF1-RF16 couplethe light from the acoustic sensors S1-S96 to the optical detectorsD1-D16 via output couplers 420. In each sensor group 401-408, a certainfraction of the light from the acoustic sensors S1-S96 is coupled to oneof the optical detectors D1-D16. The amount of light coupled into eachsensor S1-S96 is preferably chosen so as to reduce the differences inthe power level of the optical signals that are delivered to thedifferent optical detectors D1-D16. In particular, the coupling ratiosof the input couplers 420 and the output couplers 430 are selected toreduce variations in the returned optical signal levels at the detectorsD1-D16. As discussed above, a design for sensor arrays that enables thesignal levels of the optical signals returned from the sensor groups401-408 to their associated detectors D1-D16 to be similar in magnitudeis disclosed in the U.S. patent application Ser. No. 09/107,399, citedabove.

The embodiment shown in FIGS. 3 and 4A-4H includes eight sensor groupsin which no two adjacent sensors have either a common distribution fiberline or a common return fiber line. The present invention is not limitedto this arrangement. For example, sixteen sensor groups can beconfigured so that each sensor group has one of the return fibers R1-R16dedicated to it as disclosed in U.S. patent application Ser. No.09/107,399 cited above.

In accordance with the present invention, the return fiber linesRF1-RF16 as well as the distribution fiber lines DF1-DF6 need to be ableto operate in a downhole and, therefore, need to be capable offunctioning and outputting a retrievable signal at a depth in the rangeof between 5,000 and 20,000 feet below the earth's surface. As describedabove, the return fiber lines RF1-RF16 as well as the distribution fiberlines DF1-DF6 are contained within the cable 202. This cable 202 servesin part to protect the acoustic array from the harsh environment of thedownhole. In particular, the return fiber lines as well as thedistribution fiber lines must be capable of functioning within the cablewhile the temperature surrounding the cable in the range of between 100°C. and 150° C. Additionally, the return fiber lines as well as thedistribution fiber lines must be capable of functioning within the cablewhile the pressure on the cable is as much as 5,500 pounds per squareinch.

The return fiber lines RF1-RF16 as well as the distribution fiber linesDF1-DF6 must be capable of functioning within the cable when the cableis immersed in water.

Although sixteen return fiber lines are shown in FIGS. 4A-4H, the numberof return fiber lines that can be used is not restricted to sixteen. Forexample, the number of return fiber lines can be doubled to 32, asdescribed above. More generally, the number of return fiber linesemployed can range from two to more than 32

Optical Detectors

In the embodiment depicted in FIGS. 1-5, the optical detectors D1-D16output an electrical signal whose magnitude is proportional to theintensity of incident light thereon. In particular, these opticaldetectors D1-D16 output a voltage or a current responsive to theintensity of incident light. In one embodiment, the optical detectorsD1-D16 output a current responsive to the intensity of incident light,and a transimpedance amplifier is employed to convert the current outputinto a voltage.

As shown in FIGS. 3 and 4A-4H, each of the return fiber lines RF1-RF16directs light onto one of the optical detectors D1-D16. In one preferredembodiment of the present invention, each of the optical detectorsD1-D16 comprises a polarization diversity receiver to guarantee thestrongest optical interference signal is taken and processed. In thisembodiment, each of the optical detectors D1-D16 includes threephotodetectors, such as photodiodes, that sense a portion of light fromthe beam incident on the optical detector. In particular, the threephotodetectors sense three different polarizations. The processingelectronics 304 subsequently samples the signal originating from each ofthe three photodetectors and selects the photodetector that yields thestrongest signal for each acoustic channel. A polarization diversityreceiver that employs three such photodiodes is described in U.S. Pat.No. 5,852,507 to Hall, which is hereby incorporated by reference herein.

Although sixteen optical detectors D1-D16 are shown in FIG. 3, thenumber of optical detectors that can be used is not restricted tosixteen. For example, the number of optical detectors D1-D16 can bedoubled to 32, as discussed above. More generally, the number of opticaldetectors D1-D16 employed can range from two to more than 32.Preferably, however, the number of optical detectors D1-D16 willcorrespond with the number of return fiber lines.

24-Channel Digital Receiver/Demodulators (Fiber Receivers)

The 24-channel digital receiver/demodulators 604, alternatively referredto as fiber receivers are displayed in FIG. 6 described above, as wellas in FIGS. 9A-9B.

FIGS. 9A-9B depict the detector/electronics assembly 601, laser drawer638, and acoustic sensor array 602 for a second embodiment of theacoustic sensing system 100 of the present invention having 192 acousticsensors S1-S192 (not shown) and six laser sources LS1-LS6.

Such a system 100 having 192 acoustic sensors S1-S192 is shown in FIG.3B described above. The system 100 in FIG. 3B comprises 192 sensorsS1-S192 contained within two separate acoustic array cables 104 appendedto two separate downlead cables 106.

The laser sources LS1, LS2, LS3, LS4, LS5, LS6 supply twelve opticalfeed lines F1-F12, which are joined at optical couplers C1-C6. A firstset of six optical feed lines F1-F6 extend from optical couplers C1-C6to a first terminator 306 a connected to a first cable 202 a. The firstcable 202 a comprises a first downlead cable 106 a and a first acousticarray cable 104 a. The first acoustic array cable 104 a holds a firstset of 96 acoustic sensors S1-S96. A second set of six optical feedlines F7-F12 extend from optical couplers C1-C6 to a second terminator306 b connected to a second cable 202 b. This second cable 202 bcomprises a second downlead cable 106 b and a second acoustic arraycable 104 b. The second acoustic array cable 104 b holds a second set of96 acoustic sensors designated S97-S192.

The first terminator 306 a also provides a link between the firstdownlead cable 106 a and sixteen return fibers R1-R16, which are coupledto sixteen optical detectors D1-D16. The second terminator 306 b alsoprovides a link between the second downlead cable 106 b and sixteenadditional return fibers designated R17-R32, which are coupled tosixteen additional optical detectors D17-D32. Such a system 100 has sixdistribution fiber lines DF1-DF6 (not shown) and 32 return fiber linesRF1-RF32 (not shown) in each cable 202 a, 202 b. The outputs of the 32optical detectors D1-D32 are electrically connected to processingelectronics 304.

In an alternative embodiment comprising 192 acoustic sensors S1-S192,the 192 sensors S1-S192 may be contained in a single acoustic arraycable 104 attached to a downlead cable 106. Such a system 100 has sixdistribution fiber lines DF1-DF6, 32 return fiber lines RF1-RF32, and 32optical detectors D1-D32.

Either a system 100 comprising a single cable 202 or a system comprisingtwo cables 202 a, 202 b can be employed in conjunction with 192 sensorsS1-S192 and the detector/electronics assembly 601 depicted in FIGS.9A-9B. As discussed above, the 192 sensors can be contained in thesingle cable 202 or a first set of sensors S1-S96 can be containedwithin a first cable and a second set of sensors S97-S192 can becontained within second cable.

FIG. 9B shows an optical sensor array 602 comprising fiber opticsensors. This optical sensor array 602 is designated a 2×(6×16) arraybecause various configurations can be employed to accommodate 192sensors S1-S192.

In FIG. 9B, the 32 return fiber lines RF1-RF32 are separated into eightgroups having four fibers each. Each group is connected to one of the24-channel digital receiver/demodulators 604 via four of the returnfibers R1-R32. The 24-channel digital receiver/demodulators 604 comprisecircuitry formed on circuit boards, and, are hereinafter referred to as24-channel digital receiver/demodulator cards or as fiber receivercards. Each fiber receiver card 604 receives four of the return fibersR1-R32 and, accordingly, contains four of the optical detectors D1-D32to sense the light from the four return fibers. Each of the returnfibers R1-R32 contains the output of six of the acoustic sensorsS1-S192. The six outputs are modulated at different frequencies, asdescribed above.

The optical detectors D1-D32 within the fiber receiver cards 604comprise polarization diversity receivers as discussed above.Polarization diversity receivers are known in the art and one suchpolarization diversity receivers described in U.S. Pat. No. 5,852,507 toHall was cited above. In this embodiment containing a polarizationdiversity receiver, each of the optical detectors D1-D32 includes threephotodetectors, such as photodiodes, that sense respective portion oflight from the beam incident on the optical detector in accordance withthe polarization of the light. The processing electronics 304subsequently sample the signal originating from each of the threephotodetectors and selects the photodetector output that yields thestrongest signal for each acoustic channel. The output of thisphotodetector is then employed until the acoustic sensing system 100 isrecalibrated.

The output of the photodetector is directed to a transimpedanceamplifier and converted from analog to digital via an analog-to-digitalconverter. This output, now in digital form, is mixed with a sinusoidalsignal at the same modulation frequency at which the output of the sixlasers L1-L6 is modulated, ω₁, ω₂, ω₃, ω₄, ω₅, and ω₆ resulting in sixsignals herein denoted I1, I2, I3, I4, I5, and I6. The digitized outputof the photodetector is also mixed with a sinusoidal signal at twice themodulation frequency at which the output of the six lasers L1-L6 ismodulated, 2ω₁, 2ω₂, 2ω₃, 2ω₄, 2ω₅, and 2ω₆, resulting in six signalsherein denoted Q1, Q2, Q3, Q4, Q5, and Q6. These resultant signalsindividually pass through circuitry that performs decimation and throughcircuitry that provides gain.

For each of the optical detectors D1-D32, twelve signals are generated.Six signals are generated by mixing at the frequencies at which the sixlaser sources LS1-LS6 are modulated, e.g., I1-I6. Six signals aregenerated by mixing at twice the frequencies at which the six lasersources are modulated, e.g., Q1-Q6. Since each fiber receiver card 604contains four of the optical detectors D1-D32 that each receive lightfrom six laser sources LS1-LS6, then each fiber receiver card produces48 resultant signals. One set of 24, derived from demodulation at thefrequencies ω₁, ω₂, ω₃, ω₄, ω₅, ω₆ and are herein denoted I1-I24 and theother set of 24, derived from demodulation at the frequencies 2ω₁, 2ω₂,2ω₃, 2ω₄, 2ω₅, and 2ω₆ are herein denoted, Q1-Q24. The eight fiberreceiver cards 604 shown in the detector/electronics assembly 601 ofFIGS. 9A-9B produce a total of 384 such resultant signals, hereindenoted I1-I192 and Q1-Q192.

Preferably, the magnitudes of the signals resulting from mixing withsinusoidal signals having the modulation frequencies ω₁, ω₂, ω₃, ω₄, ω₅,and ω₆ are equal to the magnitudes of the corresponding signalsresulting from mixing with sinusoidal signals having the frequencies2ω₁, 2ω₂, 2ω₃, 2ω₄, 2ω₅, and 2ω₆; that is, preferably |I1|=|Q1|,|I2|=|Q2|, |I3|=|Q3| . . . |I192|=|I192|. As described above, the mixedsignals I1-I192, as well as Q1-Q196, each individually pass throughseparate circuitry that can provide gain. In this manner the mixedsignals can be set to have equal magnitude, i.e., |I1| can be set equalto |Q1|, |I2| can be set equal to |Q2|, . . . and |I192| can be setequal to |I192|.

Each fiber receiver card 604 contains two demultiplexers. Onedemultiplexer is dedicated to selecting the signals resulting frommixing with a sinusoidal signal at the frequencies ω₁, ω₂, ω₃, ω₄, ω₅,and ω₆, e.g. I1-I24, the other demultiplexer is dedicated to selectingthe signals resulting from mixing with a sinusoidal signal at thefrequencies 2ω₁, 2ω₂, 2ω₃, 2ω₄, 2ω₅, and 2ω₆, e.g. Q1-Q24. Thedemultiplexers sequentially read the 24 resultant signals, e.g. I1-I24and Q1-Q24 and pairs the signals together. In sequence, each pair ofresultant signals, i.e. I1 and Q1, I2 and Q2, . . . I24 and Q24, arethen provided as inputs to circuitry that computes the arctangent of theratio of the two inputted signals, e.g., tan⁻¹[I₁/Q₁], tan⁻¹[I₂/Q₂] . .. tan⁻¹[I₂₄/Q₂₄]. This circuitry outputs the respective phase angles φ1,φ2, . . . φ24. Each phase angle, φ1-φ24, etc., corresponds to the outputof one of the acoustic sensors S1-S24, etc. These phase angles, φ1, φ2,. . . φ24, are then provided as input to circuitry that differentiatesthe phase angles with respect to time to produce dφ1/dt, dφ2/dt, . . .dφ24/dt.

In the preferred embodiment, the arctangent circuitry outputs a 16-bitword corresponding to phase. The circuitry that performs differentiationreceives the 16-bit word and outputs a 32-bit word. This 32-bit wordcomprises two 16-bit words corresponding to the differentiated phase fortwo channels, e.g. dφ1/dt and dφ2/dt, packed into one 32-bit word. Thus,in each of the 24-channel digital receiver/demodulators 604, the resultsof two channels within the 24-channel digital receiver/demodulator arepacked together into one word and the word is outputted from thereceiver/demodulator 604.

With reference to FIGS. 9A and 9B, each 32-bit word outputted by one ofthe eight 24-channel digital receiver/demodulators 604 is coupled to oneof the eight 12-DSP elements 606 via the digital signal processorcluster local bus 902 and accompanying link ports. This 32-bit word isunpacked into two 16-bit words in the 12-DSP elements 606. Since two ofthe channels are packed together, the output of the 24-channel digitalreceiver/demodulators 604 can serve as the input for the 12-DSP elements606.

Although eight fiber receiver cards (i.e., 24-channel digitalreceiver/demodulators) 604 are shown in FIG. 9B, the number of fiberreceivers that can be used is not restricted to eight. For example, thenumber of fiber receiver cards can be reduced to four. More generally,the number of fiber receivers 604 employed can range from one to morethan eight. Preferably, however, the number of fiber receiver cards 604corresponds to the number of return fiber lines RF1-RF32 and the numberof 12-DSP cards 606.

Additionally, although each fiber receiver 604 shown in FIG. 9A contains24 channels, each channel corresponding to the output of one of theacoustic sensors S1-S192, the number of channels that can be used is notrestricted to 24.

12-DSP Cards

As discussed above, the eight 12-DSP elements 606 receive 32-bit wordsoutputted by the eight 24-channel digital receiver/demodulators 604.Each one of the 12-DSP elements 606 is coupled to one of the eight24-channel digital receiver/demodulators 604 via the digital signalprocessor cluster local bus 902 and accompanying link ports.

Each 32-bit word received by one of the 12-DSP elements 606 is unpackedinto the two component 16-bit words in the 12-DSP elements 606. Each16-bit word corresponds to the output of one of the acoustic sensorsS1-S192.

The 12-DSP elements 606 decimate the incoming signal reducing the dataflow rate of the signals received by the 12-DSP elements to a rate morecompatible with the sampling rate standard to conventional seismicrecording equipment. The word “decimate” is used herein in accordancewith its conventional usage in the art as meaning to re-sample thesignal at a lower rate to reduce the original sampling rate for asequence to a lower rate. In particular, in the preferred embodiment,the 12-DSP elements 606 receive signals from the fiber receivers at arate of 512,000 samples per second and output a signal to the CPU 610 ata rate of 500, 1,000, 2,000, or 4,000 samples per second.

More specifically, the 12-DSP elements 606 convert the 16-bit words,which were obtained from unpacking the two components of the 32-bitwords, from 16-bit fixed point words to 32-bit floating point words. Thethese 32-bit words are passed through a multi-stage finite inputresponse (FIR) filter, which serves as a low pass filter. This filterhas a symmetric impulse response and introduces no phase distortion orintroduces only linear phase distortion across the frequencies. The32-bit floating point words are converted to 32-bit fixed point wordsand then passed to a RAM (Random Access Memory) buffer before being sentto the CPU 610. Each of these words correspond to the output of one ofthe acoustic sensors S1-S192.

The 12-DSP elements 606 in the embodiment depicted in FIG. 9A haveinterfaces unique to the Analog Devices SHARC (Super HarvardArchitecture) 2106x, e.g., 21060, 21061, 21062, or 21065 DSP.

As described above, each of the 12-DSP elements 606 couples itsrespective output signal to the CPU 610 via the PCI bus 608. The PCI bus608 is a generic bus conventionally employed in personal computers. Assuch, a wide variety of hardware is readily available that interfaceswith a PCI bus 608. Consequently, as improvements are made in hardwareand electronics becomes faster, components in the detector/electronicsassembly 601 can be easily replaced with these faster PCI compatibleelectronics.

Although eight 12-DSP cards 606 are shown in FIG. 9A, the number of12-DSP cards that can be used is not restricted to eight. For example,the number of 12-DSP cards 606 can be reduced to four. More generally,the number of 12-DSP cards 606 employed can range from one to more thansixteen. Preferably, however, the number of 12-DSP cards 606 correspondsto the number of fiber receiver cards 604 and return fiber linesRF1-RF32.

Additionally, although each of the 12-DSP cards 606 shown in FIG. 9Acontains 12 outputs, each output corresponding to the output of two ofthe acoustic sensors S1-S192, the number of outputs that can be used isnot restricted to 12. The number of outputs employed can range from twoto more than 24. Preferably, however, the number of DSP outputscorresponds to one-half the number of received/demodulator channels.

CPU

The CPU 610 receives the 32-bit fixed point words corresponding to theoutput of one of the acoustic sensors S1-S192 from the RAM buffer in the12-DSP cards 606. The CPU 610 truncates the 32-bit words down to 24bits. The CPU 610 also provides any necessary scaling to comply with theSEG-D format.

Additionally, to comply with SEG-D format, the CPU 610 provides timinginformation. In particular, the CPU 610 outputs the absolute measure oftime when the processing electronics 304 received the sync signal fromthe acoustic source 130. This absolute measure of time is acquired fromthe GPS electronics 628 at the time the processing electronics 304received the sync signal. The GPS card can provide 1 part per million(ppm) accuracy for time stamping events. The CPU 610 also includes themeasure of time that lapsed between when the processing electronics 304received the sync signal and when the acoustic sensing system 100 begansampling, i.e., sensing for acoustic vibration. The CPU 610 additionallyprovides the time separation between the samples.

FIGS. 6 and 9A show the CPU 610 outputting to the recording andprocessing system 618 via the Ethernet bus 622. The signal output by theCPU 610 corresponds to the filtered differentiated phase and alsoincludes the timing information described above. This output iscompliant with conventional seismic data, and more specifically, withSEG-D format. Accordingly, the phase data, i.e., the rate of change inphase, output by the CPU 610 is readable by conventional seismic datarecording and processing equipment, which e.g., can use the phase andtiming information to determine the amplitudes of the acoustic waves 102at the sensors S1-S192.

The processing electronics 304 shown in FIGS. 6, 9A, and 9B can outputdata at a sample rate of 500 hertz (Hz), 1 kilohertz (kHz), 2 kHz, and 4kHz upon the user's selection. The output data resolution is 24 bits.Conventional systems do not provide the ability to select sample ratesof, for example, 2 and 4 kHz.

Although, the processing electronics 304 shown in FIGS. 6, 9A and 9Bprovides output in SEG-D format, the invention is not so limited. Otherdata formats can be employed, for example, SEG-Y or single precision(32-bit) ASCII. Preferably, such data formats are in conformity withconventional formats.

The CPU card 610 shown in FIG. 9A is electrically connected to a mouse904, a keyboard 906, an SVGA card 908 for display, and to a hard drive612. The CPU card 610 also has Com 1 910 and Com 2 912 ports. Asdescribed above, the CPU card 610 couples to an operator console 616 viaEthernet 620.

In the embodiment shown in FIG. 9A, the CPU couples to the 12-DSP cards606, the 16-channel A/D Auxiliary Input/Output Card 624 (denoted in FIG.6 as the Auxiliary I/O), and the GPS card 628 via the PCI bus 608. TheCPU card 610 couples to the frequency synthesizer card 632 through theISA bus 640. The CPU 610 manages the operation and interaction of thesecards.

The PCI bus 608 as well as the ISA bus 640 are generic busesconventionally employed in personal computers. As such, a wide varietyof hardware is readily available that interfaces with these buses 608,640, and in particular with the PCI bus. Consequently, as improvementsare made in hardware and electronics becomes faster, components in theprocessing electronics 304 can be easily replaced with these faster PCI(or ISA) compatible electronics.

Laser Sources

In one preferred embodiment of the invention, the lasers L1-L6 produceoptical radiation at a nominal wavelength of 1319 nanometers (nm),corresponding to an optical frequency of approximately 227 terahertz(THz) in optical fiber. The frequencies may be separated byapproximately 0.5 to 3 gigahertz (GHz) and are modulated by respectivecarriers between approximately 2 (megahertz) MHz and 7 MHz.

The lasers L1-L6 may comprise Nd:YAG lasers that are all identicalexcept for the optical frequency at which they are operated. Thetemperatures of the lasers L1-L6 are preferably adjusted so that eachlaser has a unique operating optical frequency/wavelength. Operating atdifferent optical frequencies avoids optical interference between theoptical signals from different sources in the same fiber.

Although Nd:YAG lasers operating at a nominal wavelength of 1319 nm aredescribed above as being appropriate for use as lasers L1-L6, theinvention is not so limited. Rather, other lasers and other wavelengthscan be employed in accordance with the present invention. Additionally,other modulation frequencies can be employed. The selection ofappropriate modulation frequencies is discussed more fully below.

Similarly, although six laser sources modulated at six modulationfrequencies are shown in FIG. 3, the number of laser sources that can beemployed is not restricted to six. The number of laser sources employedcan range from one to more than twelve.

More, generally, instead of employing laser sources LS1-LS6 to couplelight into the acoustic sensors S1-S192, other optical sources can beused. The optical source can be a coherent source, such as a laserdiode, or an incoherent source, such as a light emitting diode (LED) ora fiber source.

Frequency Synthesizer Card

The frequency synthesize card 632 provides waveforms to the lasersources LS1-LS6 to establish the frequencies at which the outputs of thelasers L1-L6 are modulated. The frequency synthesizer card 632 alsoprovides clock, synchronization, and timing to the fiber receivers 604for synchronizing the system 100 and phase locking the demodulators 604to the modulators M1-M6.

In the embodiment shown in FIGS. 6, 9A, and 9B, the frequencysynthesizer produces six periodic waveforms at six different frequenciesω₁, ω₂, ω₃, ω₄, ω₅, ω₆. The frequency synthesizer card sends thewaveforms at the six frequencies ω₁, ω₂, ω₃, ω₄, ω₅, ω₆ to the lasermodulation control driver card 636 in the laser drawer 638 viaelectrical line 914. The frequency synthesizer card 630 also sends thecritical timing and synchronization signals to each of the fiberreceiver cards 604. The frequency synthesizer card 630 sends thesesignals to the fiber receiver cards 604 via a plurality of shieldedsignal lines 916.

As discussed above, the frequency synthesizer card 630 sends the syncsignal and clock signal to the fiber receiver cards 604 and, from thesetwo signals, the fiber receiver cards 604 generate digital carriers atthe six modulation frequencies ω₁, ω₂, ω₃, ω₄, ω₅, ω₆ and at twice thesix modulation frequencies 2ω₁, 2ω₂, 2ω₃, 2ω₄, 2ω₅, and 2ω₆ for mixingand demodulation.

Although six frequencies are generated by the frequency synthesizer card630 shown in FIGS. 6, 9A, and 9B, the number of frequencies produced isnot restricted to six. The number of frequencies employed can range fromtwo to more than twelve. Preferably, the number of frequencies willcorrespond to the number of laser sources LS1-LS6.

Selection of Modulation Frequencies

As noted above, the signals from six sensors, e.g. S1-S6, may bemultiplexed within a single return fiber, e.g., RF1, using frequencydivision multiplexing. Due to the nonlinear nature of theinterferometer, this modulation results in signal output from theinterferometer modulated not just at the six modulation frequencies,f_(n)(=ω_(n)/2π), where n=1, . . . , 6, but also at 2f_(n), 3f_(n),4f_(n), etc. The f_(n) frequencies will be referred to as the“modulation frequencies” or “fundamental frequencies,” and the highermultiples of f_(n) will be referred to as “harmonics,” such that the2f_(n) signals are the “first harmonics,” or “harmonics of the firstorder,” the 3f_(n) signals are the “second harmonics,” or “harmonics ofthe second order,” etc. The group of N fundamental frequencies will bereferred to as the “fundamental set.” Similarly, the group of N firstharmonic frequencies will be referred to as the “first harmonic set,”and so on for the higher harmonics.

As noted above, the multiplexed intensity signal received by a givendetector may be demultiplexed by detection of the signals at f_(n) and2f_(n). For the foregoing demodulation technique to work, however, eachof the f_(n) and 2f_(n) components of the multiplexed signal must beisolated in frequency space. That is, the set of f_(n) modulationfrequencies must be selected so that no f_(n) or 2f_(n) components(i.e., the “information containing components”) overlaps with any otherfrequency component, including any of the higher harmonics. Anyinformation containing component that is overlapped in frequency spacecannot be unambiguously demodulated. As will become more clear below,this limitation complicates the selection of modulation frequencies.

Each frequency component in the multiplexed output contains signal overa bandwidth centered about the frequency. The size of the bandwidthdepends upon the frequency characteristics of the signal received by thesensor and possibly upon the frequency response of the sensor itself.Once the operating bandwidth of the frequency components is known, thevarious f_(n) values must be selected with sufficient spacing to ensurethat no overlapping results. The minimum spacing needed to avoid overlapbetween neighboring components will be referred to as Δf.

FIGS. 10A and 10B illustrate one approach to selecting frequencies so asto avoid interfering with information carrying components. The plotdepicts the multiplexed signal frequency spectrum containing acousticalinformation received simultaneously by a single detector from aplurality of acoustical sensors. The numbers represent frequency valuesin multiples of Δf. Thus, if Δf=0.5 MHz, the positions indicated as 9,10, 11, 12, and 13 correspond to actual frequencies of 4.5 MHz, 5.0 MHz,5.5 MHz, 6.0 MHz, and 6.5 MHz, respectively. The larger the selection ofΔf, the greater the possible dynamic range of the system. Thus, inpractice, Δf is selected to be as large as possible.

The multiplexed signal is depicted as a series of bullet-shapedcomponents distributed along the spectrum. The width of each componentdepicts the frequency bandwidth for that component of the signal. Thefrequency value associated with a particular component indicates thefrequency at the center of the component. Components containing theletter “F” represent fundamental frequencies. Components containing anumber represent harmonic frequencies, with the number representing theorder of the harmonic. Thus, the first order harmonics contain a “1,”the second order harmonics contain a “2,” etc. Harmonics higher thansecond order are omitted from FIGS. 10A and 10B in the interest ofclarity.

FIGS. 10A and 10B show multiplexed signal spectra for two systems inwhich the fundamental, first harmonic, and second harmonic sets do notoverlap. The five-light-source system of FIG. 10A utilizes evenly spacedmodulation frequencies at 9 Δf through 13 Δf. The spacing betweenneighboring fundamental frequencies is selected to equal Δf, thesmallest spacing allowed. FIG. 10B illustrates the analogoussix-light-source system using modulation frequencies at 11 Δf through 16Δf. This approach ensures that the fundamental components will not beinterfered with by any of the harmonics, and that the first harmonicswill not be interfered with by the fundamentals or by the second orhigher harmnonics. Since there is no overlapping of any of theinformation carrying signals, complete demodulation of the transmittedsignal is possible. This approach, however, fails to use large portionsof the frequency spectrum. For example, FIG. 10A demonstrates that thefive-light-source system makes no use of the frequencies at Δf multiplesof 0 to 8, 14 to 17, 19, 21, 23, or 25.

The highest information-containing frequency is depicted in FIGS. 10Aand 10B as a dashed vertical line. In order to simplify the electronicsneeded for processing the received signal, it is preferable to selectthis frequency to be as low as possible. FIGS. 10A and 10B illustratethat, in the absence of overlapping sets, the processing forfive-light-source and six-light-source systems must be designed tohandle frequencies of at least 26 Δf and 32 Δf, respectively.

The problem of unused frequency space associated with the approach ofFIGS. 10A and 10B is aggravated as the number of light sourcesincreases. For an N-light-source system, the lowest fundamentalfrequency, f₁, may not be chosen below (2N−1)Δf, and the processingsystem must handle the largest first harmonic frequency, 2f_(N), of(6N−4)Δf. For example, a twelve-light-source system could not do betterthan f₁=23 Δf and 2f₁₂=68 Δf.

FIGS. 11A and 11B illustrate two embodiments in accordance with oneaspect of the present invention. The embodiments maintain an equallyspaced set of fundamental frequencies starting at a lower frequency thanallowed in the non-overlapping approach of FIGS. 10A and 10B.

Comparison of FIGS. 10A and 11A indicates that for the five-light-sourcesystem the embodiment of FIG. 11A reduces the lowest fundamentalfrequency from 9 Δf to 7 Δf, while the highest first harmonic frequencyis reduced from 26 Δf to 22 Δf This lowering of frequencies causes thebeginning of the second harmonic set (at 21 Δf) to be at a lowerfrequency than the maximum frequency of the first harmonic set (at 22Δf). The overlapping of sets interleaves the individual frequencycomponents in such a manner that none of the information carryingcomponents is interfered with. In particular, the non-informationcarrying component 3f₁, at 21 Δf, is harmlessly nestled between theinformation carrying components 2f₄ and 2f₅, at 20 Δf and 22 Δf,respectively.

Similarly, a comparison of FIGS. 10B and 1B indicates that for thesix-light-source system the embodiment of FIG. 11B reduces the lowestfundamental frequency from 11 Δf to 9 Δf, while the highest firstharmonic frequency is lowered from 32 Δf, to 28 Δf. As with thefive-light-source system, the lowest second harmonic frequency isinterleaved between the two highest first harmonic frequencies, suchthat no information carrying components is interfered with.

The embodiments illustrated in FIGS. 11A and 11B may be generalized toany multiplexed system utilizing three or more light sources. For anN-light-source system, where N≧3, an embodiment includes equally spacedfundamental frequencies starting at f₁=(2N−3)Δf. For the remainingmodulation frequencies, this gives, for 1>n≧N, f_(n)=f_(n−1)+Δf

This class of embodiments results in a highest first harmonic frequencyat 2f_(N)=(6N−8)Δf. Comparing these values with the corresponding valuesabove indicates that these embodiments reduce the lowest fundamentalfrequency by 2 Δf and the highest first harmonic frequency by 4 Δfrelative to the best non-overlapping approach. TABLE I illustrates theselection of modulation frequencies associated with these embodimentsfor values of N ranging from 3 to 9.

TABLE I N Modulation Frequencies (multiples of Δf) 3 3, 4, 5 4 5, 6, 7,8 5 7, 8, 9, 10, 11 6 9, 10, 11, 12, 13, 14 7 11, 12, 13, 14, 15, 16, 178 13, 14, 15, 16, 17, 18, 19, 20 9 15, 16, 17, 18, 19, 20, 21, 22, 23

FIGS. 12 and 13 illustrate two embodiments that utilize a 2 Δf gap in anotherwise equally spaced (at Δf intervals) set of fundamentalfrequencies.

FIG. 12 shows a five-light-source embodiment with fundamentalfrequencies ranging from 6 Δf to 11 Δf, skipping an intermediateposition at 9 Δf. This selection of fundamental frequencies allows thefirst harmonic set to shift down near the fundamental set. It alsoallows the second harmonic set to substantially overlap the firstharmonic set. The second harmonic components are interleaved, however,so as not to interfere with any of the first harmonic components.

Comparison of FIGS. 10A and 12 indicates that this five-light-sourceembodiment reduces the lowest fundamental frequency from 9 Δf to 6 Δfrelative to the best non-overlapping approach, while the highest firstharmonic frequency is lowered from 26 Δf, to 22 Δf.

The embodiment illustrated in FIG. 12 may be generalized to anymultiplexed system utilizing five or more light sources. For anN-light-source system, where N≧5, an embodiment includes equally spacedfundamental frequencies starting at F₁=(2N−4)Δf, except for skipping thefrequency at 3(N−2)Δf. This gives the following modulation frequencies:f₁=(2N−4)Δf; f_(N−1)=f_(N−2)+2 Δf, f_(N)=(3N−4)Δf, and, for 1<n<N−1,f_(n)=f_(n−1)+Δf.

This class of embodiments results in a highest first harmonic frequencyat 2f_(N)=(6N−8)Δf. TABLE II illustrates the selection of modulationfrequencies associated with this embodiment for N ranging from 5 to 11.

TABLE II N Modulation Frequencies (multiples of Δf) 5 6, 7, 8, 10, 11 68, 9, 10, 11, 13, 14 7 10, 11, 12, 13, 14, 16, 17 8 12, 13, 14, 15, 16,17, 19, 20 9 14, 15, 16, 17, 18, 19, 20, 22, 23 10 16, 17, 18, 19, 20,21, 22, 23, 25, 26 11 18, 19, 20, 21, 22, 23, 24, 25, 26, 28, 29

FIG. 13 shows a six-light-source embodiment with fundamental frequenciesranging from 7 Δf to 13 Δf, skipping an intermediate position at 12 Δf.This selection of fundamental frequencies allows the first harmonic setto shift down until it abuts up against fundamental set. The secondharmonic components substantially overlap the first harmonic components,but are interleaved so as not to interfere with any of the informationcarrying components.

Comparison of FIGS. 10B and 13 indicates that this six-light-sourceembodiment reduces the lowest fundamental frequency from 11 Δf to 7 Δfrelative to the best non-overlapping approach, while the highest firstharmonic frequency is lowered from 32 Δf to 26 Δf.

The embodiment illustrated in FIG. 13 may be generalized to anymultiplexed system utilizing four light sources or six or more lightsources. For an N-light-source system, where N≧4, N≠5, an embodimentincludes equally spaced fundamental frequencies starting at f₁=(2N−5)Δf,except for skipping the position at 3(N−2)Δf. This gives the followingmodulation frequencies: f₁=(2N−5)Δf; f_(N)=f_(N−1)+2 Δf, and, for 1<n<N,f_(n)=f_(n−1)+Δf.

This class of embodiments results in a highest first harmonic frequencyat 2f_(N)=(6N−10)Δf. TABLE III illustrates the selection of modulationfrequencies associated with this embodiment for N ranging from 4 to 11.

TABLE III N Modulation Frequencies (multiples of Δf) 4 3, 4, 5, 7 6 7,8, 9, 10, 11, 13 7 9, 10, 11, 12, 13, 14, 16 8 11, 12, 13, 14, 15, 16,17, 19 9 13, 14, 15, 16, 17, 18, 19, 20, 22 10 15, 16, 17, 18, 19, 20,21, 22, 23, 25 11 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 28F

FIGS. 14A and 14B illustrate a six-light-source embodiment that utilizestwo gaps of unequal size. The embodiment uses fundamental frequencies,shown isolated in FIG. 14A for clarity, at Δf multiples of 5⅔, 7, 8, 9,10, and 12½. As shown in FIG. 14B, this embodiment results in an overlapbetween the fundamental and first harmonic sets, with the lowest firstharmonic frequency (at 11⅓ Δf) interleaved between the two highestfundamental frequencies (at 10 Δf and 12½ Δf). The third harmonic setjoins the second harmonic set in overlapping the first harmonic set. Asrequired, the interleaving of the higher harmonics avoids interferingwith any of the information carrying components.

FIGS. 15A and 15B illustrate a six-light-source embodiment that utilizesthree gaps. This embodiment uses fundamental frequencies, shown isolatedin FIG. 15A for clarity, at Δf multiples of 3, 4, 5, 7, 11, and 13. Asshown in FIG. 15B, this embodiment results in the first, second andthird harmonic sets all overlapping the fundamental set. The firstharmonic set is overlapped by higher harmonics extending out to theseventh harmonic set. Although FIG. 15B indicates that there issubstantial overlapping between different signal components (depicted bythe bands on top of other bands), none of the overlapping interfereswith the information carrying components.

The embodiment illustrated in FIGS. 15A and 15B may be generalized toany multiplexed system utilizing four or more light sources. For anN-light-source system, where N≧4, an embodiment includes fundamentalfrequencies at multiples of Δf equaling 3 and 4, followed by the nextN−2 consecutive prime numbers beginning at 5. Thus, the modulationfrequencies may be written as: f₁=3 Δf; f₂=4 Δf; and, for 2<n≦N,f_(n)=Δf, where X_(n) is the (n−2)th consecutive prime number startingat 5. TABLE IV illustrates the selection of modulation frequenciesassociated with this embodiment for different values of N.

TABLE IV N Modulation Frequencies (multiples of Δf) 4 3, 4, 5, 7 5 3, 4,5, 7, 11 6 3, 4, 5, 7, 11, 13 7 3, 4, 5, 7, 11, 13, 17 8 3, 4, 5, 7, 11,13, 17, 19 9 3, 4, 5, 7, 11, 13, 17, 19, 23 10 3, 4, 5, 7, 11, 13, 17,19, 23, 29

Although the embodiments illustrated above usually present themodulation frequencies as integer values of the minimum spacingparameter, Δf, it will be recognized by one skilled in the art that theinvention could be practiced by choosing frequencies varying slightlyfrom these integer values. The amount of variation allowed depends uponthe relative sizes of the component bandwidths and Δf. Furthermore,FIGS. 10A through 15B depict systems with component bandwidths exactlyequal to Δf. This aspect of the figures is stylistic. The embodimentspresented above include systems for which the component bandwidths arenarrower than Δf.

High Pressure High Temperature Hydrophone

In preferred embodiments of the present invention, the hydrophone sensorarray operates at pressures of at least 5,000 psi and at temperatures ofat least 130° C. More preferably, the hydrophone sensor array operatesat pressures of at least 5,000 psi and at temperatures of at least 150°C. Most preferably, the hydrophone sensor array operates at pressures ofat least 5,000 psi and at temperatures of at least 180° C.

In particularly preferred embodiments of the present invention, thehydrophone sensor array operates at pressures of at least 8,000 psi andat temperatures of at least 110° C. More preferably, the hydrophonesensor array operates at pressures of at least 8,000 psi and attemperatures of at least 150° C. Most preferably, the hydrophone sensorarray operates at pressures of at least 8,000 psi and at temperatures ofat least 180° C.

The small outer diameter of the hydrophone sensor array of the presentinvention is particularly advantageous. In preferred embodiments of thepresent invention, the outside diameter of the sensor array is no morethan 1.5 inches. In particularly preferred embodiments, the outsidediameter of the sensor array is no more than approximately 1.375 inches.In other preferred embodiments, the outside diameter of the sensor arrayis between approximately 1.375 inches and approximately 1.5 inches. Instill other preferred embodiments, the outside diameter of the sensorarray is no more than approximately 1.0 inch. The small outside diameterof the sensor array allows the hydrophone to be inserted into thedownhole casing of a well without removing the production tubing. Thesensor array may also be inserted into a length of production tubing.

The outside diameter of the hydrophone sensor array of the presentinvention is substantially uniform (±0.020 inch) over the length of thearray. The uniform outside diameter permits the array to be insertedinto a conventional grease injection head of an oil well under pressureso that pressure control of the oil well may be maintained. The outercovering of the array fits snugly in the injection head and islubricated by grease under pressure so that the array may be loweredinto the well without releasing the pressure in the well. One skilled inthe art will appreciate that a stacked fitting is advantageously appliedto the wellhead to accommodate the smaller uniform outside diameter ofthe downlead cable.

The general layout of a preferred hydrophone embodiment 1000 is shown inFIG. 16, which is capable of operating under extreme conditions such astemperatures of up to about 220° C. and pressures of 10,000 or even15,000-20,000 pounds per square inch (psi). The hydrophone may alsooperate satisfactorily under less extreme conditions such astemperatures of at least 150° C. and pressures of 8000 psi, ortemperatures of at least 130° C. and pressures of at least 6000 psi.Sensors 1002 are inserted at periodic intervals along a 1.0 inch to 1.5inch diameter (e.g., 1.25 inch diameter) cable 1004, with one suchsensor 1002 being shown in FIG. 16. Alternatively, the cable 1004 mayhave a diameter between 0.9 inch and 2.0 inches. In one preferredembodiment, the sensors are spaced almost exactly 5 feet from eachother, within a tolerance of ¼ inch. The cable 1004 includes an outersheath 1008 which surrounds a filler member 1012 that extends around thesensors 1002. In the portions of the cable 1004 away from the sensors1002, the outer sheath 1008 surrounds a core member 1016 which surroundsa plurality of tubular strands 1020 disposed around a central strengthmember 1024. These relationships are seen more clearly in the crosssectional view of the cable 1004 shown in FIG. 17.

The central strength member 1024 is located along the center of thecable 1004 and provides strength to the cable 1004 except at thoselocations where the sensors 1002 are located. The strength member 1024includes a plastic sheath 1028 that surrounds 6-8 bundles 1030, witheach bundle having 15-20 steel strands 1034 of a diameter ofapproximately 0.015 inch. The overall diameter of the strength member1024 may be {fraction (7/32)} inch. The tubular strands 1020 may be, forexample, 0.084 inch diameter Hytrel™ 5556, Hytrel™ 7246, or Hytrel™ 8238from DuPont (which have melting points and Vicat softening points of203° C., 180° C.; 218° C., 207° C.; and 223° C., 212° C., respectively).The tubular strands 1020 surround conductors or optical fibers, or thetubular strands may just be empty (filler strands) to lend structuralintegrity to the hydrophone 1000. In one particular embodiment, twelvetubular strands 1020 are used, in which two strands carry copperconductors, four strands each carry six optical fibers, and the sixremaining strands are filler strands. Such an embodiment is suitable foruse in a 6×16 array in which two optical fibers are designated asspares. The copper conductors may be used to provide electrical power toa device at the distal end of the cable 1004, e.g., a gamma tool forsensing purposes.

The core member 1016 extends along the length of the cable 1004 exceptin and around the sensors 1002. The core member 1016 may advantageouslybe Furon (0611-950 from Furon Company). In the area of each sensor 1002,the filler member 1012 is advantageously polyurethane (e.g., PRC 1547from Courtaulds Aerospace) which extends out to a diameter of 1.0 inchto hold together the components making up the sensors 1002. As such, thefiller member 1012 is formed around the sensors 1002 after the sensorshave been positioned within the cable 1004. The outer sheath 1008 may be0.1 inch thick Hytrel™ 5556, Hytrel™ 7246, or Hytrel™ 8238 and extendsalong the entire length of the cable 1004. (A high temperature,Teflon-based material such as Tefzel may be substituted for the Hytrel™materials herein.) The outer sheath 1008, the filler member 1012, andthe core member 1016 function as protective layers to protect thehydrophone 1000 (including its reference mandrel and its sensingmandrel, discussed below) from a corrosive environment. The outsidediameter of the hydrophone 1000 is preferably less than approximately1.5 inches, and more preferably is less than approximately 1 inch.

As seen in FIG. 16, the strength member 1024 is joined to a flange 1040which transfers axial load from the strength member 1024 to a stressrelief mechanism such as a plurality of stress relief wires 1050(discussed below in connection with FIG. 18) and then to a second flange1040. In this manner, the hydrophone 1000 (and in particular, thereference mandrel, the sensing mandrel, the reference fiber, and thesensing fiber, which are discussed below) are substantially isolatedfrom the axial load. The strength member 1024 is advantageouslysurrounded by a spring 1060 near that point where the strength member1024 is joined to the flange 1040 by a conventional high-pressureswaging process. The tubular strands 1020 also advantageously passthrough the spring 1060, although the strands 1020 are not shown in thisportion of FIG. 16 for the sake of clarity.

As seen in FIG. 18, the flanges 1040 are located near respective ends ofthe hydrophone 1000. The flanges 1040 may include a plurality of raisedareas 1064 around which the stress relief wires 1050 are wrapped andbetween which there are grooves (not shown in FIG. 18) that receive thetubular strands 1020. A plurality of 1-inch long spring members 1080(discussed below) support the stress relief wires 1050. The stressrelief wires 1050 advantageously cross over each other as shown in FIG.18 to form a “cage” that prevents the cable 1004 from being twistedexcessively, which could damage the sensors 1002. The stress reliefwires 1050 preferably wrap at least ⅔ of the way around the sensor 1002in the radial sense as they extend from one flange 1040 to the otherflange. With this arrangement, the stress relief wires 1050 cross overeach other between the spring members 1080 rather than on top of thespring members 1080. The flanges 1040 themselves preferably have nosharp edges or features, in order to reduce the risk of damage to thetubular strands 1020, or to the conductors or optical fibers therein.For the same reason, the stress relief wires 1050 may be Teflon coated.The hydrophone 1000 is advantageously constructed to be flexible enoughthat it can be bent to a radius of curvature of less than approximatelyfour feet.

As illustrated in FIG. 19, the sensor 1002 includes a telemetry can1104, a reference mandrel 1110, and at least one, but preferably two,sensing mandrels 1120, 1122, all of which are aligned end-to-end(coaxially) to reduce the profile of the cable 1004. This is to becontrasted with the common prior art configuration of placing thereference mandrel within the sensing mandrel. Using two sensing mandrels1120, 1122 instead of just one may result in improved sensitivity, sinceall other things being equal, using two sensing mandrels permits moresensing fiber to be used. The telemetry can 1104 has a hole 1128 thereinfor receiving a distribution fiber (bus) 1130 that carries an inputoptical signal 1132 generated by an optical source. Together, thesensors 1002 along the cable 1004 may advantageously form a sensor arraysuch as the 6×16 optical array described in the copending U.S. patentapplication Ser. No. 09/107399 entitled “Architecture for large opticalfiber array using standard 1×2 couplers,” filed Jun. 30, 1999, which ishereby incorporated by reference herein. The distribution fiber 1130 isspliced to an input telemetry coupler 1150 (see FIG. 20A), which isadvantageously located within the telemetry can 1104. A second hole 1134in the telemetry can 1104 permits passage of the distribution fiber 1130out of the telemetry can 1104 after a portion of the input opticalsignal has been tapped off by the coupler 1150. When the sensor 1002forms part of an array, the distribution fiber 1130 may beadvantageously coupled to other sensors at further locations along thearray cable 1004.

The telemetry can 1104 likewise houses an output telemetry coupler 1154coupled to a return fiber (bus) 1160. The return fiber 1160 enters thetelemetry can 1104 through a third hole 1164. As the return fiber 1160enters the telemetry can 1104, the fiber 1160 already carries outputoptical signals from sensors located distal of the sensor 1002, unlessthe sensor 1002 is the most distal sensor on a return fiber. Aperturbed, output optical signal 1168 from the sensor 1002 is coupled bythe output telemetry coupler 1154 onto the return fiber 1160. The returnfiber 1160 then passes through a fourth hole 1172 in the telemetry can1104 and may be coupled to other sensors along the cable 1004 beforebeing directed towards signal processing electronics.

The optical architecture related to the reference mandrel 1110 andsensing mandrels 1120, 1122 is now described. The input optical signaltapped off by the input telemetry coupler 1150 is directed along aninput optical fiber 1180 that passes through a hole 1184 in thetelemetry can 1104 and a hole 1188 in the reference mandrel 1110. Asshown in FIG. 20A, the input optical fiber 1180 is joined to an inputhydrophone coupler 1192. The input hydrophone coupler 1192 is locatedwithin the reference mandrel 1110 and directs a fraction of the inputoptical signal onto a reference fiber 1196. Another fraction of theinput optical signal is directed onto a sensing fiber 1198.

The reference fiber 1196 and the sensing fiber 1198 act as a referencearm and a sensing arm of an interferometer, respectively, which in FIG.20A is illustrated as being a Mach-Zehnder interferometer. The referencefiber 1196 exits a hole 1202 in the reference mandrel 1110 and forms 8“layers” around the reference manual (i.e., the reference fiber iswrapped 8 times in a close packed fashion around the reference mandrel1110 such that each loop of the reference fiber on the mandrel is incontact with an adjacent loop of the reference fiber) before reenteringthe reference mandrel 1110 through another hole 1206. The sensing fiber1198 passes out of a hole 1210 in the reference mandrel 1110 and formsone layer around the sensing mandrel 1120 before being directed to thesensing mandrel 1122, where the sensing fiber forms 4 layers. Thesensing fiber 1198 is then directed back onto the sensing mandrel 1120where the sensing fiber forms 3 additional layers, so that the sensingfiber forms a total of 4 layers on the sensing mandrel 1120. At thispoint, the sensing fiber 1198 enters a hole 1214 in the referencemandrel 1110. The reference fiber 1196 and the sensing fiber 1198 arespliced to an output hydrophone coupler 1218 (see FIG. 20A) locatedwithin the reference mandrel 1110. Light propagating to the coupler 1218from the two arms interferes at the coupler 1218. Specifically, theoutput hydrophone coupler 1218 receives an optical signal from thereference arm (reference fiber 1196) and an optical signal from thesensing arm (sensing fiber 1198), and produces an output optical signalwhich is directed onto an output optical fiber 1222. The output opticalfiber 1222 passes out of a hole 1226 in the reference mandrel 1110 andinto a hole 1230 in the telemetry can 1104. The output optical fiber1222 carries the perturbed, optical output signal and is spliced to theoutput telemetry coupler 1154 as described above.

The sensing fiber 1198 is wound in tension around the sensing mandrels1120, 1122. The sensing mandrels 1120, 1122 deform (expand and contract)in response to acoustic signals, such that the tension in the sensingfiber 1198 that surrounds the sensing mandrels is modified, thuschanging the overall length of the sensing fiber 1198. The length of thesensing fiber 1198 and thus the optical path length for opticalradiation passing through the sensing fiber 1198 is altered, which inturn affects the phase difference between the optical radiationpropagating in the reference fiber 1196 and the optical radiationpropagating in the sensing fiber 1198. In this way, the sensor 1002 actsas a Mach-Zehnder interferometer that records variations in acousticpressure. Although a preferred sensor architecture has been describedwith respect to 8 layers of fiber around the reference mandrel 1110 and4 layers of fiber around each of the sensing mandrels 1120 and 1122,utilizing a different number of layers is possible. Increasing thenumber of layers and sensing mandrels leads to greater sensitivity, butalso increases the cost. The sensor 1002 herein advantageously has ahigh scale factor of −140 dB relative to radians/micropascal.

A different interferometer configuration, e.g., Michelson or Fabry-Perotis also possible. FIG. 20B illustrates an alternative configuration,which functions as a Michelson interferometer. The input hydrophonecoupler 1192 and the output hydrophone coupler 1218 are replaced by asingle hydrophone coupler 1199 which performs both functions. At the endof the reference fiber 1196 and at the end of the sensing fiber 1198 areplaced respective reflectors 1200 a and 1200 b, thereby permittingoptical interference in the hydrophone coupler 1199. The hydrophonecoupler 1199 of this Michelson configuration is advantageously placedwithin the reference mandrel 1110.

FIG. 20C illustrates yet another alternative configuration, whichfunctions as a Fabry-Perot interferometer. In this design, there is noreference fiber 1196 or reference mandrel 1110. At the output side ofthe input telemetry coupler 1150 there is a partial reflector 1201 a.Similarly, a partial reflector 1201 b is at the input side of the outputtelemetry coupler 1154. The partial reflectors 1201 a, 1201 b form theFabry-Perot interferometer and are preferably fiber Bragg gratings. Inthis configuration, the input telemetry coupler 1150, the outputtelemetry coupler 1154, and the partial reflectors 1201 a, 1201 b areadvantageously housed within the telemetry can 1104.

The telemetry can 1104, the reference mandrel 1110, and the sensingmandrels 1120, 1122 preferably include respective main bodies 1260 a,1260 b, 1260 c, 1260 d of length 3.9 inches and diameter ofapproximately 0.48 inch as well as respective pairs of endcaps 1264 a,1266 a; 1264 b, 1266 b; 1264 c, 1266 c; 1264 d, 1266 d (discussed inmore detail below), as illustrated in FIG. 19. FIG. 21 illustrates thereference mandrel 1110 in more detail. As indicated in FIG. 19, thevarious fibers enter and exit through holes located in the endcaps 1264a, 1266 a; 1264 b, 1266 b. Fibers do not pass through any of the endcapsin the sensing mandrels 1120 and 1122. The endcaps 1264 a, 1266 a; 1264b, 1266 b; 1264 c, 1266 c; 1264 d, 1266 d (discussed in more detailbelow) preferably have a convex-shaped, hemispherical contour to helpwithstand high pressure and advantageously have diameters which areslightly larger than the diameter of their respective main bodies 1260a, 1260 b, 1260 c, 1260 d, so that the layers of fiber are confined towrap around the main body. The telemetry can 1104 is preferably ofmetallic construction, such as steel, and preferably has metallicendcaps 1264 a, 1266 a.

The reference mandrel 1110 provides a stable reference against whichoptical path length changes in the sensing arm can be determined, i.e.,the reference mandrel is substantially insensitive to acoustic signalsto reduce the effect of the acoustic signals on the reference fiber1196. To reduce deformation of the reference mandrel 1110 in response tochanges in pressure, the reference mandrel, including its endcaps 1264b, 1266 b, is advantageously made of metal, such as steel. On the otherhand, the walls of the reference mandrel 1110 are preferably kept thin,e.g., to about 0.05 inch, to reduce the profile of the device, whichtends to allow some pressure response from the reference mandrel 1110(i.e., some flexing of the reference mandrel) in response to acousticsignals. To compensate for this and reduce the sensitivity of thereference mandrel 1110 to acoustic signals, a cover 1270 may beadvantageously placed over the reference fiber 1196 (shown in cutaway inFIG. 21), in which the cover 1270 advantageously extends between and issealed to the endcaps 1264 b, 1266 b. An air cavity at, for example, 1atmosphere may be formed between the cover 1270 and the reference fiber1196 to act as a pressure buffer. The outside diameter (O.D.) of thecover 1270 may be about 0.53 inches. An adhesive such as Torrseal™ maybe used to seal the cover 1270, in which the adhesive is allowed to flowover the endcaps 1264 b, 1266 b as well as those portions of thereference fiber 1196 extending approximately 6 mm from either end of themain body 1260 b. The cover 1270 thus isolates the reference fiber 1196from ambient pressure, thereby improving the stability of the referencemandrel 1110 as an interferometric reference source. The referencemandrel 1110 may be partially potted to hold the input and outputhydrophone couplers 1192, 1218 in place, or alternatively, glue may beused.

The sensing mandrels 1120, 1122 are made of a high temperature materialwhich, when it is subjected to high pressure, is stiff enough that themandrels do not crack due to deformation. On the other hand, themandrels 1120, 1122 are flexible enough that they bend (undergo strain)in response to acoustic pressure, without buckling under hydrostaticpressure. Further, this high temperature material has a stiffness thatremains relatively stable at temperatures over 200° C. Two plastics thatare suitable for this purpose are Torlon™ (specifically Torlon™ 5030)and Celazole™. Of the two, Celazole™ is preferred because it is stableup to higher temperatures, and because its slightly lower stiffnessresults in greater sensor sensitivity. Further, Celazole™ exhibits lowercreep under hydrostatic loading. This latter feature is important in thecontext of interferometers, since changes as small as a few tenths of apercent in the length of the sensing fiber 1198 can significantlydiminish the noise performance of the hydrophone sensor 1002. BothTorlon™ and Celazole™ are advantageous over the prior art materials,which include thin wall aluminum and polycarbonate. Polycarbonate, forexample, is in general not suitable for work at temperatures above about105° C. Torlon™ and Celazole™, however, are suitable for work atpressures of at least 10,000 or even 15,000-20,000 pounds per squareinch and temperatures of at least 220° C.

Torlon™ 5030 is a polyamideimide and has a tensile strength of 24,000psi, a tensile modulus of elasticity of 1,200,000 psi, an elongation of4%, a flexural strength of 36,000 psi, a flexural modulus of elasticityof 1,000,000 psi, a compressive strength (10% deformation) of 38,000psi, a compressive modulus of elasticity of 600,000 psi, all of whichare measured at 73° F. Further, Torlon™ 5030 has a coefficient of linearexpansion of 1.0×10⁻⁵ in/in/° F., a heat deflection temperature at 264psi of 539° F., and a maximum continuous service temperature in air of500° F. (All values are approximate.)

Celazole™ PBI (polybenzimidazole) has a tensile strength of 23,000 psi,a tensile modulus of elasticity of 850,000 psi, an elongation of 3%, aflexural strength of 32,000 psi, a flexural modulus of elasticity of950,000 psi, a compressive strength (10% deformation) of 50,000 psi, acompressive modulus of elasticity of 900,000 psi, all of which aremeasured at 73° F. Further, Celazole™ 5030 has a coefficient of linearexpansion of 1.3×10⁻⁵ in/in/° F., a heat deflection temperature at 264psi of 800° F., and a maximum continuous service temperature in air of750° F. (All values are approximate.)

The endcaps 1264 a, 1266 a; 1264 b, 1266 b; 1264 c, 1266 c; 1264 d, 1266d are advantageously hemispherical so that the telemetry can 1104, thereference mandrel 1110, and the sensing mandrels 1120, 1122 flex moreuniformly when subjected to pressure and can thereby withstand thehigher pressures that may be encountered in the down hole applicationsdisclosed herein, which may easily exceed 3000-4000 psi. Thishemispherical design avoids stress being concentrated in small areas andis to be contrasted with the prior art design of cylindrical endcapswhich can fail under hydrostatic pressure.

The endcaps 1264 a, 1266 a; 1264 b, 1266 b; 1264 c, 1266 c; 1264 d, 1266d (shown in their assembled configuration in FIGS. 19 and 21) areadvantageously all the same shape, which is illustrated by the crosssectional representation of a preferred endcap 1264 a shown in FIG. 22.The outside diameter (O.D.) of the endcap 1264 a (designated as “C” inFIG. 22) is advantageously approximately 0.477 inches. The endcap 1264 ahas a lip 1280 that has an O.D. of about 0.206 inches (designated as “B”in FIG. 22) and an I.D. of about 0.206 inches (designated as “A” in FIG.22). The lips 1280 of the endcaps 1264 a, 1264 b, 1264 c, 1264 d aredesigned to slip within and mate with their respective main bodies 1260a, 1260 b, 1260 c, 1260 d. Each of the endcaps 1264 a, 1266 a; 1264 b,1266 b; 1264 c, 1266 c; 1264 d, 1266 d is preferably of the samematerial as its corresponding main body 1260 a, 1260 b, 1260 c, 1260 d.Thus, the endcaps 1264 a, 1266 a, 1264 b, 1266 b are preferablymetallic. The endcaps 1264 c, 1266 c, 1264 d, 1266 d are preferablyeither Torlon™ or Celazole™ to match the construction of theirrespective main bodies 1260 c and 1260 d.

FIG. 19 shows that there are three pairs of oppositely facing endcaps:1266 a, 1264 b; 1266 b, 1264 c; and 1266 c, 1264 d. Each of these pairsof endcaps is advantageously surrounded with a resilient, pliablematerial (not shown in FIGS. 16, 18, 19, 21, 22 for the sake of clarity)such as polyurethane (PRC 1547 is preferred) which forms a flexibleinterlink. For example, polyurethane forms a flexible interlink 1296(see FIG. 23) that joins the endcap 1266 a of the telemetry can 1104 tothe endcap 1264 b of the reference mandrel 1110. The interlink 1296includes grooves 1300, 1304 therein for accepting the optical fibers1180 and 1222. Likewise, another flexible interlink (not shown) joinsthe reference mandrel 1110 to the sensing mandrel 1120, and yet anotherflexible interlink (not shown) joins the sensing mandrels 1120, 1122 toeach other. Each of these additional interlinks has grooves therein foraccepting the sensing fiber 1198, thereby protecting the sensing fiber1198 from damage.

In the case of the telemetry can 1104 and the reference mandrel 1110,the interlink grooves 1300, 1304 are aligned at both ends of theflexible interlink 1296 with a hole in an endcap, e.g., the groove 1300may be used to route the input optical fiber 1180 from the hole 1184 inthe telemetry can 1104 to the hole 1188 in the reference mandrel 1110.Similarly, the groove 1304 may be used to route the output optical fiber1222 from the hole 1226 in the reference mandrel 1110 to the hole 1230in the telemetry can 1104. (The endcaps 1264 c, 1266 c, 1264 d of thesensing mandrels 1120, 1122 advantageously use grooves (not shown)rather than holes for receiving the sensing fiber 1198.) The interlink1296 is thicker between the endcaps 1266 a, 1264 b than it is near theendcaps as a result of the hemispherical shapes of the endcaps, whichhelps reduce any localized stresses that might break the fibers 1180,1222. Further, the grooves 1300 and 1304 are advantageously cut todifferent depths so that the fibers 1180 and 1222 lie in differentplanes, i.e., the fibers 1180 and 1222 cross over and are adjacent eachother without “pinching” each other. Specifically, the respective depthsof the two grooves 1300, 1304 may be selected to differ by at least thewidth of one of the fibers 1180, 1222. For example, the groove 1300 maybe cut one fiber width deeper than groove 1304, with the input opticalfiber 1180 (which carries the input optical signal) being laid downfirst during assembly. With the input optical fiber 1180 in place, theoutput optical fiber 1222 (which carries the perturbed, output opticalsignal) may then be placed down in the groove 1304 so that the outputoptical fiber 1222 crosses over the input optical fiber 1180.

The flexible interlinks, such as the interlink 1296, permit the cable1004 to be bent and flexed in the normal course of operations, e.g.,while the cable 1004 is being reeled in or out, without breakage ordamage to any of the fibers. Likewise, the grooves 1300, 1304, as wellas the grooves in the other interlinks (not shown), are multi-layered sothat when the cable 1004 is bent, the fibers will not damage each other.The grooves 1300, 1304 allow the fibers 1180, 1222 to be routed with awell controlled pitch across a flexible portion of the hydrophone 1000,namely, the interlink 1296. The grooves 1300, 1304 also ensure that thefibers 1180, 1222 maintain this pitch while entering and exiting theinterlink 1296. In one preferred embodiment, this pitch is approximately⅓ inch, i.e., the fiber 1180 (1222, 1198) makes one complete revolutionaround the interlink 1296 for every ⅓ inch along the length of theinterlink. The fiber 1180 (1222, 1198) preferably forms an angle of atleast about 72 degrees with the axis of the cable 1004 (or hydrophone1000) if the interlink 1296 has a diameter of 0.48 inch (or a smallerangle for a smaller diameter interlink, and a larger angle for a largerdiameter interlink). Thus, the fiber 1180 (1222, 1198) preferably formsan angle θ with the longitudinal axis of the hydrophone 1000 such thatcos θ times the diameter of the hydrophone (or interlink 1296) is lessthan about 0.18. The interlinks 1296 may advantageously be 1 inch long,corresponding to 3 complete revolutions of the fiber 1180 (1222).

The interlinks may be constructed by taking a pair of endcaps (e.g.,1266 a, 1264 b) and aligning them so that they are oppositely facingeach other, in accordance with FIGS. 19 and 23. Short segments of wire(not shown) such as copper wires may then be inserted into each of theholes 1184, 1230 of the endcap 1266 a and the holes 1188, 1226 of theendcap 1264 b. With the wire segments in place, a mold (not shown) maybe used to form polyurethane around the pair of oppositely facingendcaps 1266 a, 1264 b, during which time the wire segments keeppolyurethane out of the holes 1184, 1230, 1188, 1226. The wire segmentsmay then be removed and the grooves 1300, 1304 cut in the polyurethane,so that the grooves 1300, 1304 are properly aligned with theirrespective holes in the endcaps 1266 a, 1264 b.

The telemetry can 1104 is preferably assembled by beginning with twopieces (not shown) corresponding to the two halves of a main body thatwould be formed when the main body is cut lengthwise. Next, the fibers1130, 1160 are cut, passed through their corresponding pairs of holes(1128, 1134 and 1164, 1172, respectively) in the endcap 1264 a andspliced to the couplers 1150, 1154. The couplers 1150, 1154 along withtheir corresponding splices, as well as the fibers 1130, 1160 may thenbe placed into one of the halves. The fibers 1180 and 1222, in turn, maythen be passed through their respective holes 1184, 1230 in theinterlink 1296, specifically through the endcap 1266 a (see FIG. 23).The interlink 1296 and the endcap 1264 a are then be glued to the mainbody 1260 a with epoxy, and the fibers 1130, 1168 are glued into theirrespective holes using epoxy. (The epoxy herein may be a hightemperature aluminum filled epoxy such as Cotronics 454B.) The interlink1296 is then dipped in polyurethane to form a thin layer 1308 thatencapsulates the fibers 1130, 1160 to keep the fibers in a fixedposition (i.e., the fibers are “hard potted”). The telemetry can 1104may then be partially potted using epoxy (or glue may be used) to keepthe couplers 1150, 1154 and their corresponding splices from beingjostled and damaged during operation. The two halves may then be sealedtogether at ambient pressure with epoxy to form the telemetry can 1104,which is capable of withstanding hydrostatic pressure and protecting thecouplers 1150, 1154 which are positioned therein.

The reference mandrel 1110 and the sensing mandrels 1120, 1122 areadvantageously assembled in a similar fashion, except that it is notnecessary to begin the assembly procedure with halves of a main body.(In the case of the reference mandrel 1110, the hydrophone couplers1192, 1218 may be inserted into the reference mandrel through one of itsends before the reference mandrel is sealed with its endcaps. Thesensing mandrels 1120, 1122, on the other hand, do not house opticalcomponents.) The reference fiber 1196 and the sensing fiber 1198 arewrapped around the reference mandrel and the sensing mandrel,respectively. The reference mandrel 1110 and the sensing mandrels 1120,1122 are likewise sealed at ambient pressure and can withstand verylarge hydrostatic pressures. In the case of the reference mandrel 1110,the cover 1270 may be placed over the reference mandrel 1110 to act as apressure buffer, as discussed above.

Once the assembly of the sensor 1002 (see FIGS. 16 and 19) is complete,the interlinks 1296 of the sensor 1002 are advantageously surrounded bythe spring members 1080 (see FIG. 18) for additional protection againstthe strains and stresses that may be encountered during deployment andoperation of the hydrophone 1000. Following assembly of the flanges 1040and their associated stress relief wires 1050 around the sensor 1002, amaterial such as polyurethane (e.g., the PRC 1547 from CourtauldsAerospace, discussed above) may be molded around the sensor 1002, thespring members 1080, the spring 1060, the flanges 1040, and the stressrelief wires 1050 to form the filler member 1012 so that the hydrophone1000 is well shielded from the harsh chemical and mechanical conditionsassociated with down hole applications. As a result of this moldingprocedure, the interlinks 1296 are well surrounded by polyurethane,since polyurethane is also advantageously used to construct theinterlinks, as discussed above. In this manner, the fibers 1180, 1230,1196, 1198 are embedded in flexible interlinks 1296 which have the pitchand tension necessary to survive the bending encountered duringdeployment and handling of the cable 1004.

The molding procedures disclosed herein (in connection with, forexample, the interlink 1296 or the hydrophone 1000) may be performed byplacing a mold around the object to be encased and then pulling a vacuumon that object. The object may be heated to 140° F. for 10-15 minutesbefore polyurethane is injected around it. After injecting polyurethanearound the object, the vacuum may be maintained for 15-20 minutes todegas the polyurethane. The polyurethane may then be cured for 14 hoursat 40-70 psi and 140° F. before the mold is removed.

The use of polyurethane in the various components disclosed herein(e.g., the filler member 1012 and the interlink 1296) limits use of thehydrophone 1000 to temperatures up to about 150° C. Teflon or Viton maybe substituted for polyurethane, however, and these materials may beused up to about 220° C. The optical couplers and adhesives herein mayfunction up to temperatures of 200° C. or even 220° C.

System Performance

The acoustic sensing system 100 of the present invention may includenumerous acoustic sensors S1-S192. The embodiments described aboveinclude 96 and 192 sensors S1-S192, respectively, as well as 96 and 192channels in the processing electronics 304 for processing the output ofthe 96 or 192 sensors. Having a large number of sensors S1-S192 offers asignificant improvement over prior art systems. For example, having alarge number of sensors S1-S192 increases the potential resolution ofmeasurements such as cross-well tomography and also dramatically reducesthe time required to complete a geological survey.

The acoustic sensing system 100 of the present invention offers otheradvantages over the prior art. TABLE V provides a summary of theperformance and specifications of the acoustic sensing system 100described above comprising 96 fiber optic sensors S1-S96. The acousticsensing array 602 of the present invention, however, is not limited to96 or 192 sensors S1-S192 but may include as many as 400 sensors.

As discussed above, the acoustic array 602 is small enough to fit intoproduction tubing. The cable 202 shown in FIG. 2 can be inserted inproduction tubing having an inner diameter of two inches and even inproduction tubing having an inner diameter of 1.25 inches. The cable 202in the embodiment described above with 96 sensors has an outer diameterof 1.22 inches and includes armoring. Thus, the acoustic array 602 canbe inserted in the production tubing in the casing of a well 118 ratherthan requiring removal of the production tubing to fit the cable in thecasing.

The acoustic sensing system 100 of the present invention is ruggedenough to operate in the harsh downhole environment. The cable 202 canbe inserted in a well 118 to a depth of over 10,000 feet where thetemperature is over 150° C. and the pressure is over 5,500 pounds persquare inch.

The acoustic sensing system 100 of the present invention has a largeenough bandwidth to perform real time sensing of the acoustic wave,including processing the output of the acoustic sensors S1-S192 andoutputting data in conventional seismic format. Since the acousticsensors are optical sensors, they do not limit the bandwidth of thesystem. Rather, the bandwidth is limited by the bandwidth of theprocessing electronics 304. However, the processing electronics 304 isfast enough to measure the acoustic vibration produced by an acousticsource 130 and permit viewing of the results soon thereafter.Consequently, if data are to be acquired, processed, and outputted inreal time and in a format that the surveyor can read, the surveyor canmodify the survey based on the results being generated. For example, ifthe data appears to indicate the possible presence of an in-placereserve, the acoustic source 130 and/or acoustic sensor array 602 couldbe repositioned for further investigation.

In contrast, limitations on speed and bandwidth prevent conventionalacoustic sensor arrays from achieving real time processing. Rather, oncemeasurements are taken, data is recorded on magnetic tape and istransported to a location away from the well 118 or drill site forprocessing.

In addition to being fast, the acoustic sensor system 100 of the presentinvention has a low acoustic noise floor. In particular, the integratedRMS acoustic noise over the detection bandwidth is 0.1 microbar RMS.

The acoustic sensor system 100 of the present invention also has a widedynamic range. Large voltage outputs for small acoustic signals enablethe system to sense and record small amplitude acoustic waves 102. Atthe same time, the system is able to sense and record large amplitudeacoustic waves 102. Specifically, the embodiment described above having96 sensors S1-S96 has an instantaneous dynamic range of 132 decibels(dB) for the acoustic band ranging from 1 Hz to 400 Hz and has aninstantaneous dynamic range of 120 dB for the acoustic band ranging from401 Hz to 1000 Hz.

TABLE V PERFORMANCE CHARACTERISTICS CAPABILITY Number of AcousticChannels 96 Expandable to 192 Lead Cable Length 10,000 feet Array CableLength 500 feet Array Cable Diameter 1.22 inches Includes armoringOperating Pressure in excess of 5500 p.s.i. Operating Temperature inexcess of 150° C. Noise Floor 0.1 mbar RMS Instantaneous Dynamic Range132 dB Minimum from 1 Hz to 400 Hz 120 dB Minimum from 401 Hz to 1000 HzDistortion −80 dB Crosstalk −85 dB Acoustic Passband 1 Hz to 1440 HzRipple +/−0.2 dB Channel-to-channel +/−0.34 dB Output Data Sample Rate 4kHz, 2 kHz, 1 kHz, and Selectable 500 Hz Output Data Format SEG-D Rev. 2Output Data Resolution 24 bits Fixed point Auxiliary Channels 16 InputSignal Amplitude 10 VDC (0 to peak) Maximum Input Frequency 1.5 kHzSample Rate 4 kHz Resolution 16 bits External Sync 10 msecBi-directional TTL or switch closure Electronics Cabinet 48″ x 19″ x17″; AC powered less than 250 lbs. GPS Capability Included 1575 MHzantenna Gamma Tool Included

The acoustic sensor system 100 minimizes crosstalk between signals of adifferent wavelength. The crosstalk of the system having 96 sensorsS1-S96 is −85 dB.

The acoustic sensor system 100 also minimizes distortion. The distortionof the system having 96 sensors S1-S96 is −80 dB.

The acoustic sensor system 100 has an acoustic bandpass between 1 Hz and1440 Hz. Accordingly, frequency components between 1 Hz to 1440 Hz ofthe acoustic wave are sensed by the system 100. The acoustic sensorsystem 100 outputs data in SEG-D REV.2 format, a conventional formatcomplying with standards set by the Society of Exploration Geophysiciststhat is well know in the art. The acoustic sensor system 100 also canoutput data at a sample rate of 500 Hz, 1 kHz, 2 kHz, and 4 kHz upon theuser's selection. The output data resolution is 24 bits.

As described above, the system 100 can accept auxiliary channels. Theembodiment described above having 96 sensors S1-S96 can accept sixteensingle-ended auxiliary channels or eight differential auxiliarychannels. These auxiliary channels have a maximum input frequency of 1.5kHz. These channels are sampled at a rate of 4 kHz and with a resolutionof sixteen bits.

The system 100 also accepts an external sync pulse. The embodimentdescribed above having 96 sensors S1-S96 accepts a 10-millisecond longexternal sync pulse. This sync pulse can be generated usingbidirectional TTL (i.e., with active pull-up and active pull-down) orswitch closure (i.e., active pull-down with resistive pull-up).

As described above, the acoustic sensing system 100 preferably comprisesa GPS system 628. The acoustic sensing system 100 additionally maycomprise a gamma tool. Gamma tools, which are well known in the art, areused to measure the depth of the cable by counting markers on the casingas discussed above.

One additional advantage provided by the acoustic sensing system 100 ofthe present invention is that this system is significantly lesssensitive to tube waves than conventional systems. A tube wave, as iswell known in the art, corresponds to acoustic waves traveling up anddown the borehole 124, either through the metal casing or through waterin the bore hole. During data acquisition, the acoustic sensing system100 of the present invention advantageously is less affected by tubewaves than conventional acoustic sensing systems.

Although the acoustic sensing system 100 of the present invention hasbeen described in the downhole environment for the purpose ofgeophysical surveys designed to locate oil reserves, its use is not solimited. This acoustic sensing system 100 of the present invention maybe otherwise employed to measure acoustic vibrations at a series ofremote locations.

More generally, the present invention may be embodied in other specificforms without departing from the essential characteristics describedherein. The embodiments described above are to be considered in allrespects as illustrative only and not restrictive in any manner. Thescope of the invention is, therefore, indicated by the following claimsrather than the foregoing description. Any and all changes which comewithin the meaning and range of equivalency of the claims are to beconsidered in their scope.

What is claimed is:
 1. An electronic instrument for processing aplurality of optical signals produced by a plurality of optical sensorsthat sense subterranean acoustic waves, said electronics comprising: aplurality of optical detectors that convert said optical signals intoelectrical signals; and an interface that outputs a signal derived fromsaid electrical signal in seismic data format; wherein said opticalsignals when outputted from said optical sensors comprise modulatedlight such that said output electrical signals output from said opticaldetectors comprise electrical signals modulated at a plurality ofmodulation frequencies.
 2. The system of claim 1, wherein said opticalsensors comprise fiber optic sensors.
 3. The system of claim 1, whereinsaid seismic data format comprise SEG-D format.
 4. The system of claim1, wherein said seismic data format comprise SEG-Y format.
 5. The systemof claim 1, further comprising at least one demodulator that demodulatessaid modulated electrical signals.
 6. The system of claim 1, whereinsaid demodulator mixes said modulated electrical signals with periodicwaveforms having frequencies corresponding to said modulationfrequencies and twice said modulation frequencies.
 7. The system ofclaim 1 wherein said optical sensors are contained in an acoustic arraycable.
 8. The system of claim 7, wherein said cable is lowered to abottom surface of a body of water.
 9. The system of claim 7, whereinsaid optical sensors comprise an optical sensor array.
 10. The system ofclaim 7, wherein said optical sensors in said acoustic array cable arecapable of functioning in a downhole environment.
 11. A The system ofclaim 10, wherein said cable is lowered into a well.
 12. The system ofclaim 11, wherein said cable is lowered into an oil well.
 13. Anelectronic instrument for processing a plurality of modulated opticalsignals produced by a plurality of optical sensors that sensesubterranean acoustic waves, said electronics comprising: a plurality ofoptical detectors that convert said optical signals into modulatedelectrical signals; at least one mixer for mixing at least one of saidmodulated electrical signals with a carrier to generate a mixed signal;a filter component that is applied to the mixed signal to obtain ademodulated signal; and an interface that outputs a signal derived fromsaid demodulated signal in seismic data format.
 14. The electronicinstrument of claim 13, wherein said optical detectors comprisepolarization diversity receivers.
 15. The electronic instrument of claim13, wherein said carrier comprises a periodic waveform having afrequency that is substantially the same as the frequency at which saidmodulated electrical signal is modulated.
 16. The electronic instrumentof claim 13, further comprising at least one additional mixer for mixingeach of said modulated electrical signals with a carrier comprising aperiodic waveform having a frequency that is substantially the same astwice the frequency at which said modulated electrical signal ismodulated.
 17. An electronic instrument for processing a plurality ofmodulated optical signals produced by a plurality of optical sensorsthat sense subterranean acoustic waves, said electronics comprising: aplurality of optical detectors that convert said optical signals intomodulated electrical signals; a plurality of channels with each channelcomprising one mixer for mixing one of said modulated electrical signalswith a carrier to generate a first mixed signal; a first filtercomponent that is applied to the first mixed signal to obtain a firstdemodulated signal; and an interface that outputs a signal derived fromsaid first demodulated signal in seismic data format.
 18. The electronicinstrument of claim 17, wherein said optical detectors comprisepolarization diversity receivers.
 19. The electronic instrument of claim17, wherein said carrier comprises a periodic waveform having afrequency that is substantially the same as the frequency at which saidmodulated electrical signal is modulated.
 20. The system of claim 17,analog-to-digital converters for converting said modulated electricalsignals output from said optical detectors into digital signals.
 21. Theelectronic instrument of claim 18, further comprising at least oneadditional mixer in each of said channels for mixing each of saidmodulated electrical signals with a carrier comprising a periodicwaveform having a frequency that is substantially the same as twice thefrequency at which said modulated electrical signal is modulated therebygenerating a second mixed signal; and a second filter component that isapplied to the second mixed signal to obtain a second demodulatedsignal.
 22. The electronic instrument of claim 21, further comprising aninverse tangent circuit that outputs an inverse tangent of the ratio ofsaid first demodulated signal and said second demodulated signal. 23.The electronic instrument of claim 22, further comprising adifferentiator circuit that differentiates said inverse tangent outputfrom said inverse tangent circuit thereby producing a differentiatedsignal.
 24. The system of claim 23, further comprising digital signalprocessor circuitry for decimating said differentiated signal.
 25. Theelectronic instrument of claim 23, further comprising processingelectronics that outputs said differentiated signal in seismic dataformat.
 26. The electronic instrument of claim 25, wherein saidprocessing electronics comprises a central processing unit.
 27. Thesystem of claim 25, wherein said seismic data format comprise SEG-Dformat.
 28. The system of claim 25, wherein said seismic data formatcomprise SEG-Y format.
 29. An electronic instrument for processing aplurality of optical signals produced by a plurality of optical sensorsthat sense subterranean acoustic waves, said electronics comprising:means for converting said optical signals into electrical signals; andmeans for outputting a signal derived from said electrical signal inseismic data format; wherein said optical signals when outputted fromsaid optical sensors comprise modulated light such that said signalderived from said electrical signal comprises electrical signalsmodulated at a plurality of modulation frequencies.
 30. A method forprocessing a plurality of modulated optical signals produced by aplurality of optical sensors that sense subterranean acoustic waves,said method comprising: converting said optical signals into modulatedelectrical signals; mixing at least one of said modulated electricalsignals with a carrier to generate a mixed signal; filtering the mixedsignal to obtain a demodulated signal; and outputting a signal derivedfrom said demodulated signal in seismic data format.